Dart driveshaft needed. Any info would help

I have read that about the only way to mate a 68+ transmission (904 or 727) with an earlier engine (slant or SB) is to use an early torque converter and associated input shaft. Alternatives are to have the later torque converter snout machined smaller (or a custom converter) or machine a bigger hole in your crankshaft.

I would think that rebuilding your cable-shift transmission would be simplest since you can keep your shifter. I recently got a rebuild kit for both of mine for $35 on ebay. He had several listed, so search.

Using a 727 would rob power. I have also heard that you need to beat up the sheet-metal lip on the body to fit one in an early-A and it is still tight. I recall the 727 uses a larger flexplate too.

A driveline shop could fairly easily cut your existing driveshaft shorter. A lot of people don't like keeping the ball & trunnion joint since parts are rare and expensive, though I got spares for mine reasonably on ebay recently. Several have had a shop fabricate a whole new driveshaft, using a sliding joint common in trucks. Didn't cost too much. Search for posts here.