swapping to a big block from a 6 cylinder 4 speed - The 1947 - Present Chevrolet & GMC Truck Message Board Network

kxnomore · 09-14-2011, 10:26 PM

Forgive me if this has been posted a million times...

The truck is a 1969 C-10 shortbed. It currently has a 350 installed in what seems to be the original 6 cylinder mounts. I believe this because the engine is very low and far back in the engine compartment. But the clutch and transmission works great, and looks to be in the original location, based on how the bellhousing is bolted to the crossmember... My plan is to install a 402 I have from a parts truck, using new big block mounts and towers, and to keep the 4 speed.

What challenges are there in keeping the riveted in crossmember from the original straight 6 and 4 speed transmission? do you have to cut that crossmember out and would the transmission have to be moved backwards or forwards?

The main concern I have is the transmission crossmember that is riveted in, seems to be necessary to support the bellhousing etc, and seems like it would be tough to install the big block without taking it off. Do the big block towers actually locate the engine in exactly the right location? What have you guys done when keeping the original granny low 4 speed for this conversion?

Any suggestions would be very helpful, thanks!

71swb4x4 · 09-14-2011, 10:32 PM

I can't help with the BB location but I can tell you the crossmember isn't a big issue. Pop the rivets out and bolt it in where you want it if it's not in the right place. Most of the trucks I have had didn't have the original rivets anymore because someone before me had removed them and replaced with bolts.

rsavage · 09-15-2011, 07:17 AM

Note on above: There isn't a transmission crossmember on a truck with a 4 speed - it is a bellhousing crossmember and I don't think it can be moved. The transmission hangs from the bellhousing.
I haven't done a big block install with a 4 speed truck transmission so can't help you with specifics, but I don't see what the difference would be other than mounts. I am sure someone who has done it will chip in.

OJ1988 · 09-15-2011, 07:20 AM

I would drop it in those 6 cyl mounts. You may have to do a little massaging on the firewall to clear your distributer, and valve covers, but it would be a lot easier than moving the trans crosmember, and redoing the clutch linkage, driveshaft etc...

09-14-2011, 10:26 PM	#1
kxnomore Registered User Join Date: Dec 2006 Location: CA Posts: 331	swapping to a big block from a 6 cylinder 4 speed Forgive me if this has been posted a million times... The truck is a 1969 C-10 shortbed. It currently has a 350 installed in what seems to be the original 6 cylinder mounts. I believe this because the engine is very low and far back in the engine compartment. But the clutch and transmission works great, and looks to be in the original location, based on how the bellhousing is bolted to the crossmember... My plan is to install a 402 I have from a parts truck, using new big block mounts and towers, and to keep the 4 speed. What challenges are there in keeping the riveted in crossmember from the original straight 6 and 4 speed transmission? do you have to cut that crossmember out and would the transmission have to be moved backwards or forwards? The main concern I have is the transmission crossmember that is riveted in, seems to be necessary to support the bellhousing etc, and seems like it would be tough to install the big block without taking it off. Do the big block towers actually locate the engine in exactly the right location? What have you guys done when keeping the original granny low 4 speed for this conversion? Any suggestions would be very helpful, thanks!

09-14-2011, 10:32 PM	#2
71swb4x4 Senior Member Join Date: Nov 2003 Location: Brookings, SD Posts: 10,497	Re: swapping to a big block from a 6 cylinder 4 speed I can't help with the BB location but I can tell you the crossmember isn't a big issue. Pop the rivets out and bolt it in where you want it if it's not in the right place. Most of the trucks I have had didn't have the original rivets anymore because someone before me had removed them and replaced with bolts. __________________ Some people are like slinkies, they aren't good for anything, but you can't help but smile when you see one tumble down the stairs.

09-15-2011, 07:17 AM	#3
rsavage Registered User Join Date: Aug 2009 Location: Alden NY Posts: 2,705	Re: swapping to a big block from a 6 cylinder 4 speed Note on above: There isn't a transmission crossmember on a truck with a 4 speed - it is a bellhousing crossmember and I don't think it can be moved. The transmission hangs from the bellhousing. I haven't done a big block install with a 4 speed truck transmission so can't help you with specifics, but I don't see what the difference would be other than mounts. I am sure someone who has done it will chip in. __________________ 1961 C1 Corvette 1959 El Camino 350 TPI, 9" 4 w disc 69 Blazer K5 - sold July '20 2021 Durango RT 5.7

09-15-2011, 07:20 AM	#4
OJ1988 Just a car guy Join Date: Feb 2006 Location: New Washington, IN Posts: 991	Re: swapping to a big block from a 6 cylinder 4 speed I would drop it in those 6 cyl mounts. You may have to do a little massaging on the firewall to clear your distributer, and valve covers, but it would be a lot easier than moving the trans crosmember, and redoing the clutch linkage, driveshaft etc... __________________ 1955 belair Blown Pro Street cruiser 1971 Camaro Pro Touring 1973 Vega Drag Car 1970 C20 tow rig



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