Using a 302/AOD in about the same position as the original flathead V8.

Rear axle is in the same location.

What is the approximate length of driveshaft I need?

I was going to go to the junk yard and find one slightly longer than I need and have it cut down to the right size. Obviously harder if I get one too small.

 

I have a 289/C4 combo and I think I ended up around 54" u-joint to u-joint.

 

I have a 302/AOD in my truck, and mine came to 49 1/2 from u-joint to u-joint, that gives about 1 inch of "slide" in the slip yoke. Got a salvage shaft from an 83 Crown Vic and had it shortened.

 

I agree on the Crown Vic shaft. I used a engine and trans from a 90 Crown Vic. Shaft was plenty long then had it shortened.