Rebuild a 350 or rebuild a 350 to the 383 stroker?

I have a 1990 Chevy K1500 truck with a 350 5.7 engine with over 230,000 miles and a automatic R400 transmission (i think its a R400). Engine is leaking oil somewhere. Mechanics couldn't find the leak with just a basic inspection. It is coming from the back and near bottom of the engine i believe. I just had the transmission rebuilt and was wondering how much it would cost to have the engine rebuilt by a mechanic? Also how much would it cost if I rebuilt it myself? How much extra would it cost to rebuild it to a 383 stroker? I am by no means a "expert" mechanic so is this possible for me to do? Would a 383 stroker work well with the R400 transmission or will I have problems there? Any and all help is appreciated!