69 C10 stepside trans/drive shaft questions

[Mr. M3]

I recently traded for this 69 C10 short stepside and it came without a trans and driveshaft. I picked up 2 700R4's recently and was considering putting one behind the I6 as had been suggested previously. The questions being:
1-Driveshaft length. Has anyone done something of a similar configuration? Would you have dimensions for the driveshaft?
2-A 700R4 behind an I6. Will that pair up correctly? Will I need to modify anything to make this work?
3-Future considerations. I don't plan to keep the I6 in the truck forever. I would like to keep it as simple as possible for a later swap. What I have in mind is a 350/TH350, or possibly a 383/TH350. What are your thoughts on this?

And again thanks in advance for any help!