My truck is an original OEM V8 and Automatic transmission vehicle that has been fitted with a late model 350sbc/700r setup. The transmission is bolted to the bellhousing original crossmember at the sides of the converter housing. This arrangement prevent the converter cover from being attached at the transmission bottom and exposes the converter and starter to the road. I want to install the correct rear transmission mount/crossmember. I have not been successful on finding information on the bellhousing crossmember. It has been mentioned that this factory crossmember is removable (not riveted) since this truck was an original automatic. It appears the previous owner lowered the crossmember by one bolt hole to facilitate the 700r install. Can this crossmember be removed without impacting the frame, cab mounts, or front spring mounts as these components are in the same location. Would the frame require boxing or other reinforcement such as duplicating the crossmember ends and bolting back into original position?
The original crossmember is a riveted unit, and removing it would make the frame much less stable. I have a TH400 in my truck behind a 454, so the problem is slightly different because of the bellhousing design. I originally had snubbers on both sides where the original mount holes are located. My actual transmission mount is at the rear on a crossmember that I fabricated.. I cut the original crossmember down to make room where the top of the crossmember was interfering with the stock full sized torque converter. I did this by cutting off the top where the crossmember was interfering, removing enough material from the remaining crossmember, and then welded the top back on so that it is now a thinner unit and does not interfere with the torque converter. This way, it is slightly thinner, but still looks original. The two extra crossmembers, one for the rear of the ******, and one for the overdrive, have added back more than enough stability. This arrangememt has worked well for me since I did the installation in 1971. Bob
My 57 had a 283 and a hydro in it originally , as I recollect it did have a bolt in center section with the outer ears rivited to the frame .You could remove the original center section and fabricate your own crossmember using the original bolt holes. Squirrel care to chime in?
squirrel finally woke up and got out of bed. You can remove the crossmember, and install some type of crossmember under the rear of the transmission, and some type of crossmember under the engine to use engine side mounts. Then it will work fine, there are lots of trucks done like this. If you try to use some type of engine mount that cantilevers off the top of the frame, then you would need to box the frame there. So use some type of crossmember under the engine. That's the only thing I've seen over the years, that was a problem. The TransDapt type crossmembers work. https://www.summitracing.com/parts/trd-4840/overview/
oh....it won't really make any difference...the new rear crossmember won't be anywhere near as strong as the one you remove. And it will also be a bolt in thing.
Thanks for the replies. A general question regarding boxing. Is there a rule of thumb on how long the plate is in relation to its height? 1:1 1.5:1 2:1?
Hmmm....I think I said you don't need to box it, if you add new crossmembers that attach to the side of the frame. I would not box part of the channel frame on one of these trucks unless there was a very good reason to do so. they are designed to twist. The attaching points for stuff is pretty well thought out in the original design.
The center section of the rear crossmember was bolted in on the GMC trucks. I think it was made removable was because many of them that had the Pontiac engines also used the 4 speed hydro trans.
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