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Switch from 307 to 350

Discussion in 'Chevy Trucks' started by 75gmck25, Nov 30, 2003.

  1. 75gmck25

    75gmck25 Senior Member
    Messages: 119

    On my '75 K25 I finally got around to scraping the dirt off the engine casting numbers and found it was #3970020, which is apparently a 307, not a 350. I knew the engine and trans were replaced by the previous owner, but it was still supposed to be a 350. The engine came out of a Monte Carlo, or similar vehicle.

    The carb on the engine is a 4 barrel Quadrajet #7045583, so it is probably not original to the engine. They may have moved some of accessories from the old 350 over to the 307, so I think I have an odd match. If this carb is from a Chevy, its would be a '75 California emissions carb, but I don't think it should have been on a GMC truck with an auto transmission.

    My 307 is bolted up to a TH350 (not the original TH400 that my truck came with). Will a Goodwrench 350 bolt in place of the 307, or are there motor mount or bolt pattern differences?

    How much other trouble will I have if I make a switch to a 350? My truck is 8400 GVW, so the only emissions equipment is a vapor recovery tank.


  2. ksland

    ksland Senior Member
    from ma
    Messages: 420

    Everything is a direct swap. EVERYTHING!:nod:

    EXCEPT maybe the flex plate, not sure on the balancing differences.
  3. phoenix827

    phoenix827 Member
    Messages: 98

    Ksland is right, for the most part, anything smallblock chevy is interchangeable! Got to say tho, if it was me, I would keep the 307, IIRC the power is about the same. and it's just a little bit different. Like me! lol
  4. David Smith

    David Smith Member
    Messages: 48

    All parts are interchangeable for what you want to do. Including the flex plate.

    Did same thing (more or less) a few months ago...went from a 305 to a 350
  5. ratlover

    ratlover PlowSite.com Addict
    from IL
    Messages: 1,325

    307 is a bolt in deal EXCEPT the motor mounts. You will need to get mounts for a regular SBC but they will still bolt to the same spot on the frame.....307 mounts are just a wee bit different hight. But the person that swaped it probably left the 350 mounts in.....most times a 307 or regular sbc mount will work if you are swaping them but it aint right and will tilt your motor up or down a bit from were it should be depending on if you are going to a 307 or from it.