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TH350 to TH400 swap

Discussion in 'Chevy Trucks' started by 75gmck25, Dec 21, 2003.

  1. 75gmck25

    75gmck25 Senior Member
    Messages: 119

    My truck ('75 GMC K25) was originally equipped with a TH400, 350, and NP203. At some point the previous owner blew the engine and swapped in a 307 (73 vintage) and a TH350. I plan to put in a Goodwrench 350, and I'm considering looking for a TH400 to get the truck back to the original setup.

    Since the TH350 and TH400 are different lengths, I'm trying to figure out how they swapped in the 307 and TH350. It seems like they would have needed to also swap both driveshafts, but I doubt they would have gone to that much trouble. Were there different adapters for the NP203 to trans (TH350 vs TH400) that would have been used to change the length?

    Also, would it be easier if I switched from the TH350 to a TH700R4? I wouldn't be any stronger, but the overdrive would be nice.


  2. JMR

    JMR Senior Member
    Messages: 567

    I can't answer your question about going from a TH350 to a TH400. I do know however that the 700R4 & the TH400 are the same relative length and swapping between the two is easy. I replaced the 700R4 in one of my plow truck with a TH400. No driveshaft mods. Pretty much bolted right in.
  3. ShiznityZ28

    ShiznityZ28 Junior Member
    Messages: 7

    you will have to shorten the ds to go to a th400 any ds place can do it for around 100.00 .700r4 would be nice but will not be neer as strong as a th400 or th350