I'm looking at different types of shafts for use in my longtail build and found a supplier that can get me whatever I want-my BIL He works there and can get stuff for cost-anywhere from 50-60% off regular price. It looks like 3/4" is the standard for diameter with mud motor props? If so, I should be able to get a decent shaft for about $25-40 depending on length and material. My boat will have a 20" transom when finished-how does this affect shaft length? I'm assuming I'd want to go as long as possible (72")?

3/4" in dia and 72" in length.

That is what I made mine and I had a 20" transom. Worked fine.

get 3/4" cold rolled round stock 72" long, machine one end with 3/4 x10 threads and key yhe other for your yoke. Easily can be done with $35