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I want to replace the TH 350 in my 1979 Corvette with the 200-4R overdrive transmission. The 200-4R mounting pad is located further rearward than the TH350 pad is. Does anyone know if an extension bracket of some kind is available for this kind of swap? Thanks in advance for any ideas.
I want to replace the TH 350 in my 1979 Corvette with the 200-4R overdrive transmission. The 200-4R mounting pad is located further rearward than the TH350 pad is. Does anyone know if an extension bracket of some kind is available for this kind of swap? Thanks in advance for any ideas.
None that I've heard of; but worth looking deeper.
Fortunate for you, your C3 w/ auto trans has a bolted-in & readily-removable trans Xmember. Remove it and modify it.
Or replace it w/ one of several bolt-in, ready-made options available for 200-4R.
In my 79, the transmission crossmember is removable, and the winning strategy is to replace this with a modern crossmember that allows the exhaust to pass under, instead of through the two holes.
But as part of my TKX upgrade, I ended up with a piece that replaces the flange on the stock crossmember, and allows for a TKX to mount. Here it is at Silver Sport Transmission:
kennethj!
the 200-4R uses the same mounting location as the Turbo 400. It will bolt right in if you have the standard crossmember (not sure fi the 1979 had already gone to the goofball crossmember). SO - if you have the standard 2 hole crossmember from the 68 forward then it will bolt right in as far as the rear crossmember. Go take a look at your crossmember. If it is a flat-top crossmember with 2 holes then check from underneath and see if there are 2 holes where the turbo 400 would have mounted.
I bolted it where my Muncie was..... because even the welded-in manual crossmembers came with the 2 holes up top ..... but no matter, you're better off getting an aftermarket crossmember where the exhaust passes under....
Below you can see my old crossmember and the 2 holes on top that the THM-400/200-4R will bolt onto. I ended making my own crossmember so that is what you are seeing here.... when I was beginning.
Originally Posted by kennethj!
I want to replace the TH 350 in my 1979 Corvette with the 200-4R overdrive transmission. The 200-4R mounting pad is located further rearward than the TH350 pad is. Does anyone know if an extension bracket of some kind is available for this kind of swap? Thanks in advance for any ideas.
Last edited by carriljc; Mar 23, 2026 at 02:39 PM.
I think the last year for the TH-400 was 1977. My 79 has that extra piece (pictured above) for the TH-350, and now for the TKX. But the two holes are there in the 79's crossmember, too. Perhaps all the OP needs to do is remove the smaller extension bracket entirely to mate the 200-4R.
My 78 has a removable crossmember. As Carriljc said above, that is all you need. And as Bikespace explained you just need to remove that little bracket ( the one on the right). The holes in the crossmember should line up perfectly with the trans. I have side pipes so the exhaust location wasnt an issue.
Yep. he should be able to do it. AND if his exhaust swept around the THM-350 it may even sweep around the 200-4R.
Originally Posted by Bikespace
I think the last year for the TH-400 was 1977. My 79 has that extra piece (pictured above) for the TH-350, and now for the TKX. But the two holes are there in the 79's crossmember, too. Perhaps all the OP needs to do is remove the smaller extension bracket entirely to mate the 200-4R.
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Transmission swap
