Re: GMC 350 into a ford Ranger
different maybe, but I put a 396/th400 in my 81 dodge d150(front sump 429 wouldnt fit-darn it). I used a set of bigblock 69 camaro frame stands and made steel adapter plates to bolt to stands/frame(didnt want to drill/weaken crossmember), the tranny crossmember needed two new holes drilled to bolt to frame, and another adapter plate for the rearmount. the th400 was longer than the dodge, so just cut/welded driveshaft, had local shop rebalance it. Moog(I think) makes adapter u joints to mix and match about anything- and cheep. Even cut/reworked the tailgate to say "dodge rat" instead of ram...was no hotrod, but torky enough to have some fun and overall was a perfect truck motor- drove it for 10 years. It finally started to rust, I pulled the BBC, and gave the chassis to a guy I used to work with, he put a smallblock back in and as far as I know its still on the road somewhere...
Being its a (albeit-light) truck, I personally wouldnt be too worried about the frame- now if you plan on loading the bed full and heading to the strip, you might be in trouble, but have you ever looked at how flimsy a mustang front end is? My 69's frontend looks weaker than any true frame I've ever seen, yet Ive seen pics of bigblock cars doing wheelstands...I bet if you tried it with door open the car would break in two! If I ever get my 429 in it, I'm staying with smallish tires.
A word of caution- Echeck might get you- if not applicable in your area, may still come back to haunt you if you sell it. by doing conversion, YOU are removing OEM pollution controls- if possible be sure to carryover all the GM smog crap if its a newer year motor- or pull motor out before even thinking of selling. It could trade hands several times, but if emissions EVER become an issue, you could get sued by owner(but I didnt know it was chevy powered?), or possibly even fined by state, but Ive never heard of them pushing it that far.
Confidence: that feeling you have just before you realize the mess you've gotten yourself into...