I had a 56 Studebaker wagon with a fairly strong 350. Started with a TH350 and 3:55 8 inch rear. 80 MPH was about 3400 RPM, ran warm with AC on. Switched to 700R4, ran 2500 RPM and was quieter and ran cool. That sold me on 700R4's.
Put a 700R4 in my 71 Chevy Pickup. Had it built for towing. Between my truck and trailer (full), they weight 11,000 lbs+. Works flawlessly.
That said, I agree that I would rather just buy a core and install one tranny. TV cable hookup is CRITICAL.
With my Stewed, the 350 had a 9" tail, was the same length as the 700, so I had no driveshaft issues.