Small Transmision Hump to Large hump
 

My 1966 GMC started as a 3 speed on the column truck with an inline 6. I found a 4 speed out of a 1966 that I would like to use. How much of a job is changing to a large hump setup if i do not have the portion of the large hump that is a part of the firewall. I could fabricate the part of the floorboard to raise the hump without much of an issue, but I do not know how much additional clearance is needed at the firewall. I plan to stitch weld the new panel in place.

Randy Kaneer

Re: Small Transmision Hump to Large hump
 

You need this from the firewall. You cut one like this then remove the hump side at this seam that you can see on the back. Pretty easy really.

Re: Small Transmision Hump to Large hump
 

Another pic.

Re: Small Transmision Hump to Large hump
 

Well I just realized you are in Florida. You may have trouble finding that part. If so let me know. Barry

Re: Small Transmision Hump to Large hump
 

Thanks Barry. Great photos. I just ran across an article (by Craig Ueltzen, Edited by Jolly) titled "Upgrading a SM-318 3-speed to a SM-420 4-speed" on 6066gmcguy.com. I will let you Know if i have trouble finding a donor firewall section of the large hump.
Thanks. Randy Kaneer

Re: Small Transmision Hump to Large hump
 

Will the NP833 4 speed O/D transmission from 80-87 bolt up to an inline 6 cylinder and fit under the small transmission hump in my 1966 GMC.
Thanks.
Randy Kaneer

Re: Small Transmision Hump to Large hump
 

Ok . Yes it will bolt to a 230-250-292. With a 67-90 bell housing. Yes it will fit under the hump. But you need a tranny crossmember. The bell housings don't have side mounts. And you need to shim the crossmember down about a 1/2 inch if the cab mounts are old. It's a little tight if the mounts are old and sagged. It's a good setup even though Chrysler built it. BC