Engine/Trans Advice

[nac593]

Hi, I'm new here but i've been watching the forum for a while. I have a 69 shortbed that needs a new engine. I am pretty sure i'm getting a new gm crate engine. Just the basic 350 longblock. So here's where I could use some advice. I would like to get this 700 transmission I've been reading about with the overdrive. I'll be using the truck as a daily driver. But my dad says I can have his trans from his chevy dually. TH400 I think? Sounds a lot cheaper to use my dads, but it's been sitting for probably 7 years. Is the torque converter and the other parts still usable? Would it be less headache to just buy the new trans and get it over with? My dad said all I could do is put it in and see if it works. I want a reliable truck. By the way, I'll be switching from the 3 speed manual to automatic. Also I read you have to shorten the driveshaft with the 700. Also with the 400 , or no? Any advice would be appreciated. Thanks!