Going from 6 to 8... - The 1947 - Present Chevrolet & GMC Truck Message Board Network

The Blue Pig · 08-15-2002, 04:12 AM

Hey, I was just wondering what would be involved total in switching from a 250 and a 4 speed to a small block and a 350 or 400 automatic. Anyone ever taken out a 250 or 292 and put in a small or big block? I notice that the motor mounts are further up with a V-8 than they are with an inline 6. I also notice an extra hole in the frame where it may move up, but not a hole in the crossmember, do you need a different crossmember? I know the basic stuff like new tranny crossmember for an automatic, but I was looking the the hidden stuff that may come up and bite ya in the middle of the project. Crap like maybe stiffer springs in the front, or something like that. Thanks. BTW, the reason I ask, I have this 71 with a 250 and a granny tranny, want a small block and a 400.

chevyman 68 C/10 · 08-15-2002, 05:15 AM

The crossmamber is different for auto than standard. You will need a shifter or column from an auto truck and the mounting brackets for the engine

chevyman 68 C/10 · 08-15-2002, 05:18 AM

Sorry you will need a radiator if yours is not at least a 3 core and a shroud to direct the air through the radiator. (full shroud, not the puny 6 cyl. version)

The Blue Pig · 08-15-2002, 04:08 PM

I was thinking of just going with a B&M megashifter, I like them better than the star shifters, the star shifters take a while to figure out without taken em out and throwing them away. I thought there were V-8's with 1 core radiators, I know about the shroud, but I guess I would have to upgrade anyway. So I would have to change the crossmember then? And the motor mount things?

bigvinnie · 08-15-2002, 04:56 PM

even if you could use the 6 cyl. motor mounts, I'd replace with new anyway.... your radiator will need to be plummed for an auto unless you're going with an external cooler, and you'll want a full smallblock shroud... you'll want V8 springs for the front also. May have to have the driveshaft modified as well...

Estanley · 08-15-2002, 05:04 PM

The engine towers are different and if you are going to run a 6" tail trans you may need a longer drive shaft. My truck was a 250 I6 with a three speed manual and it will soon be a 350/350. I had to get a longer drive shaft (59 1/2"). Your 4 speed may be longer though. I just bought springs from Early Classic and they said that the only difference in the springs is big block or small block so yours should be OK.

Hope this helps.

O'yeah dont forget trans cooling. If you are going to get a new radiator you may want to get a 4 core with trans internal cooler.

Estanley · 08-15-2002, 05:08 PM

Looks like you beat me to it bigvinnie . You must type faster than me.

08-15-2002, 04:12 AM	#1
The Blue Pig Thread Wizard Since 2002 Join Date: Apr 2002 Location: Fairbanks, AK Posts: 102	Going from 6 to 8... Hey, I was just wondering what would be involved total in switching from a 250 and a 4 speed to a small block and a 350 or 400 automatic. Anyone ever taken out a 250 or 292 and put in a small or big block? I notice that the motor mounts are further up with a V-8 than they are with an inline 6. I also notice an extra hole in the frame where it may move up, but not a hole in the crossmember, do you need a different crossmember? I know the basic stuff like new tranny crossmember for an automatic, but I was looking the the hidden stuff that may come up and bite ya in the middle of the project. Crap like maybe stiffer springs in the front, or something like that. Thanks. BTW, the reason I ask, I have this 71 with a 250 and a granny tranny, want a small block and a 400. __________________ --70 C-10 Longbed --71 C-10 Longbed --72 K-20 Longbed --74 C-10 Shortbed --86 Toyota Tercel (His Driver) --05 Chevy Trailblazer (Her Driver)

08-15-2002, 05:15 AM	#2
chevyman 68 C/10 Registered User Join Date: Feb 2002 Location: Gilmer, Texas USA Posts: 313	The crossmamber is different for auto than standard. You will need a shifter or column from an auto truck and the mounting brackets for the engine __________________ 69 Chevy short fleet side soon to have LT1 and 4L60E pb. ps cruise and air. 68 Chevy short fleet side 400 sb 5 inch chop top pb. ps. pw. 350 auto and shaved handles 68 anniversary no engine long wheel base 400 auto FOR SALE 68 Chevy fleet side long wheel base complete parts truck.

08-15-2002, 05:18 AM	#3
chevyman 68 C/10 Registered User Join Date: Feb 2002 Location: Gilmer, Texas USA Posts: 313	Sorry you will need a radiator if yours is not at least a 3 core and a shroud to direct the air through the radiator. (full shroud, not the puny 6 cyl. version) __________________ 69 Chevy short fleet side soon to have LT1 and 4L60E pb. ps cruise and air. 68 Chevy short fleet side 400 sb 5 inch chop top pb. ps. pw. 350 auto and shaved handles 68 anniversary no engine long wheel base 400 auto FOR SALE 68 Chevy fleet side long wheel base complete parts truck.

08-15-2002, 04:08 PM	#4
The Blue Pig Thread Wizard Since 2002 Join Date: Apr 2002 Location: Fairbanks, AK Posts: 102	I was thinking of just going with a B&M megashifter, I like them better than the star shifters, the star shifters take a while to figure out without taken em out and throwing them away. I thought there were V-8's with 1 core radiators, I know about the shroud, but I guess I would have to upgrade anyway. So I would have to change the crossmember then? And the motor mount things? __________________ --70 C-10 Longbed --71 C-10 Longbed --72 K-20 Longbed --74 C-10 Shortbed --86 Toyota Tercel (His Driver) --05 Chevy Trailblazer (Her Driver)

08-15-2002, 04:56 PM	#5
bigvinnie Registered User Join Date: Aug 2001 Location: escondido, ca 92026 Posts: 1,222	even if you could use the 6 cyl. motor mounts, I'd replace with new anyway.... your radiator will need to be plummed for an auto unless you're going with an external cooler, and you'll want a full smallblock shroud... you'll want V8 springs for the front also. May have to have the driveshaft modified as well... __________________ '72 short step, 350, 700R4, tilt, ps, pdb, a/c, lowered coils, etc., other work in progress... San Diego, CA 72 Stepside Project

08-15-2002, 05:04 PM	#6
Estanley Registered User Join Date: Nov 2001 Location: Eatonton, GA, USA Posts: 191	The engine towers are different and if you are going to run a 6" tail trans you may need a longer drive shaft. My truck was a 250 I6 with a three speed manual and it will soon be a 350/350. I had to get a longer drive shaft (59 1/2"). Your 4 speed may be longer though. I just bought springs from Early Classic and they said that the only difference in the springs is big block or small block so yours should be OK. Hope this helps. O'yeah dont forget trans cooling. If you are going to get a new radiator you may want to get a 4 core with trans internal cooler.

08-15-2002, 05:08 PM	#7
Estanley Registered User Join Date: Nov 2001 Location: Eatonton, GA, USA Posts: 191	Looks like you beat me to it bigvinnie . You must type faster than me.



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