The crossmember is easy, Have you already determined the bellhousing/flexplate, shift linkage, driveshaft yoke/length and overall adapting procedure to a 305? With all the hearsay, urban legend stories and different "adapters", I would sure mock it up or take your adapters and try them on a 700r4....... and try it on your motor before buying a bunch of stuff and find out it won't work. Unless, of course, you know what you are doing.
Buying things on ebay for a V6 from someone who says it fits is no guarantee. Little is posted about 700r4 to V6. Check with Lewis Matkin, Ed Snyder, BigBlockV6 and GMCNUT. I have yet to read a trusted 700r4 to V6 story. But I could have missed it.
Anyway, I like
this kind because they are modifiable and bolt on. There are several versions and many sellers of this tubular type.
My experience (not V6) is: I buy a running trans for $100-300, get it rebuilt for $300-400. Sometimes I take a chance and run it without rebuild, just refreshed. There are lots of very experienced, retired or health issue or Military Vet, trans mechanics who work from home.
The 700r4 is a very good choice. The
theory would be that all these "Chevy" transmission's have the same bolt patterns for tranny and torque converter, so adapter parts will work for them all.