Question 65 Cuda

If you swap in a later trans, among other things, you would need to rig up a shifter and replace the ball and trunnion driveshaft.

I would just use the original trans, rebuild it if it needs it, with the adapter ring junkyardhero mentioned above. They used to be available, may still be, but one could be machined without much trouble. https://www.forabodiesonly.com/mopar/threads/late-to-early-converter-adapter-ring-specs.182542/

The original trans would fit the car better than anything else.

If you don't like the adapter ring idea, you could spend $$$ for a new converter for your original trans, may have to change input shaft and a couple other things. Transmission shops that specialize in Mopars could fix you up with what you need. For a stock 318, I wouldn't bother.