Q&A / Tech

Mailbag: Swapping a TH-350 Transmission for a TH-700-R4

Q: I have a 1941 Ford sedan with a small block Chevy. I would like to replace the TH-350 transmission with a TH-700-R4. How much difference is there in the physical size of the transmissions? Will I need to change the transmission mount? Would the driveshaft need modification?

A: The TH-350 is 27¾ inches in total length and 20¾ inches long from the front of the transmission to the mounting pad. The TH-700-R4 is 30 inches in total length and 22¼ inches from the front to the mounting pad. To use the TH-700, you will have to move the transmission mount crossmember back 1¼ inches, and the driveshaft will have to be shortened 2¼ inches. The driveshaft yokes are the same on both transmissions.

This is another in a series of weekly Q&A Mailbag sessions with Summit Racing‘s tech department, in which there are hundreds more. Click here to see them all

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  1. I thought because the 700 is longer the drive shaft would be shorter

  2. if the 700r is longer, would the transmission not have to be shortened?

  3. I think you might mean the driveshsft would need shortened if the yokes are the same and the transmission is longer.

  4. I had to cut 3 1/4 inches off of my driveshaft

  5. OnAllCylinders says:

    Correct, the driveshaft should be shortened. We apologize for the typo and have made the correction!

  6. fred alvis says:

    do NOT use a tranny out of a c-4 vette , they don’t have a crossmember mounting pad.

  7. Use a 200-4R and you’ll have better ratios and not have to cut the driveshaft.

  8. ^ 200-4r is the same size as the th350, just have to make new mounts. Put one in my 79 Trans Am, use a servo from a grand national.

  9. is a turbo 400 with a gearvenders od better that the 4l80?

    • OnAllCylinders says:

      No one on staff here has installed Gear Vendors Overdrive before. Unfortunately, we’ll have to withhold judgment.

  10. Jim Davidson says:

    Not sure about thise measurements pist here unless there are different dimensions because I took a 700r4 out out of my 3rd gen. Camaro and put a turbo 350 in they did say it had 9” tailshaft I used factory cross member and the stick driveshaft was the same length

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