1970 SWB TH400 swap - The 1947 - Present Chevrolet & GMC Truck Message Board Network

70gmcswb · 03-05-2015, 06:29 PM

im new to the forum. i have a 1970 GMC SWB. I picked up a 350 with a TH400 that had both been rebuilt with only 1200 miles on them. the truck had a th350 in it. ovbiously the yoke doesnt match and i believe it is a little bit longer as well. i have a 2 piece driveline. my question is, can anyone tell me what needs to be done or point me in the right direction? I have tried many different combos on google but i get a bunch of info that i do not need! thank you for any help!

70gmcswb · 03-07-2015, 01:06 AM

Bump for some help?

bluek5 · 03-07-2015, 02:49 AM

Try this link http://67-72chevytrucks.com/vboard/s...d.php?t=390542 post #4 has some specifications. Your drive line will need to be shortened, yoke replaced, and the cross member will need to be moved back as well. You will also need to come up with a kick down switch and wiring. Hopefully someone will chime in who has done this swap. Hope this helps.
Paul

**desertnick69** · 03-08-2015, 04:23 AM

I don't know that all th400 have a kick down. My 69 C/20 does not, It has a modified vacuum line that goed to the modulator. It works just fine.

**Accelo** · 03-08-2015, 01:43 PM

One note of caution here. When you purchase a 400 there are three different tail shaft links and at least 4 different yolks for the driveline. Some have a bolt on yolk and the slide is built into the 2 piece drive shaft. Other are made to slide. Look for the drilled and tapped output shaft to identify the stationary yolk style 400. Be sure to get a 400 with the slide style yolk. You will need a adapter style yolk to go from the larger u-joint to the smaller one currently on your truck. As Blue5K mentioned the front half of the drive line will need to be shortened. Using the same parts you have and just cutting and welding the driveline is the most cost effective method of converting.

All 400 have the electric kick down. It is not required but will make the down shifting more accurate and the truck will kick ass when it needs to. Conversion kits are available for the carburetor style switches. I recommend converting to a new style truck gas pedal and using the factory switch that mounts to the pedal inside the cab.
All the peddles have the provision for the switch. Only the OEM trucks with the 400 actually had the switch mounted there.

70gmcswb · 03-09-2015, 05:11 PM

Quote:

Originally Posted by bluek5

Try this link http://67-72chevytrucks.com/vboard/s...d.php?t=390542 post #4 has some specifications. Your drive line will need to be shortened, yoke replaced, and the cross member will need to be moved back as well. You will also need to come up with a kick down switch and wiring. Hopefully someone will chime in who has done this swap. Hope this helps.
Paul

thanks for the info!

03-05-2015, 06:29 PM	#1
70gmcswb Registered User Join Date: Mar 2015 Location: puyallup, washington Posts: 4	1970 SWB TH400 swap im new to the forum. i have a 1970 GMC SWB. I picked up a 350 with a TH400 that had both been rebuilt with only 1200 miles on them. the truck had a th350 in it. ovbiously the yoke doesnt match and i believe it is a little bit longer as well. i have a 2 piece driveline. my question is, can anyone tell me what needs to be done or point me in the right direction? I have tried many different combos on google but i get a bunch of info that i do not need! thank you for any help!

03-07-2015, 01:06 AM	#2
70gmcswb Registered User Join Date: Mar 2015 Location: puyallup, washington Posts: 4	Re: 1970 SWB TH400 swap Bump for some help?

03-07-2015, 02:49 AM	#3
bluek5 Registered User Join Date: Apr 2012 Location: Lehi,Utah Posts: 1,330	Re: 1970 SWB TH400 swap Try this link http://67-72chevytrucks.com/vboard/s...d.php?t=390542 post #4 has some specifications. Your drive line will need to be shortened, yoke replaced, and the cross member will need to be moved back as well. You will also need to come up with a kick down switch and wiring. Hopefully someone will chime in who has done this swap. Hope this helps. Paul

03-08-2015, 01:43 PM	#5
Accelo Senior Member Join Date: Mar 2015 Location: washington Posts: 2,224	Re: 1970 SWB TH400 swap One note of caution here. When you purchase a 400 there are three different tail shaft links and at least 4 different yolks for the driveline. Some have a bolt on yolk and the slide is built into the 2 piece drive shaft. Other are made to slide. Look for the drilled and tapped output shaft to identify the stationary yolk style 400. Be sure to get a 400 with the slide style yolk. You will need a adapter style yolk to go from the larger u-joint to the smaller one currently on your truck. As Blue5K mentioned the front half of the drive line will need to be shortened. Using the same parts you have and just cutting and welding the driveline is the most cost effective method of converting. All 400 have the electric kick down. It is not required but will make the down shifting more accurate and the truck will kick ass when it needs to. Conversion kits are available for the carburetor style switches. I recommend converting to a new style truck gas pedal and using the factory switch that mounts to the pedal inside the cab. All the peddles have the provision for the switch. Only the OEM trucks with the 400 actually had the switch mounted there.

03-08-2015, 04:23 AM	#4
desertnick69 Senior Member Join Date: Jan 2012 Location: Phoenix, Arizona Posts: 499	Re: 1970 SWB TH400 swap I don't know that all th400 have a kick down. My 69 C/20 does not, It has a modified vacuum line that goed to the modulator. It works just fine.



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