Jim, the 200-4R will handle the output of the OHC just fine in stock form. The later ones have the factory upgrades needed to survive behind a V-8 without problems; the earlier ones had some issues as did the 700-R4.
200-4Rs are used behind just about everything now, including 572s. The 200-4R is easier to mod to handle that kind of power than the 700-R4 and has the dual bolt pattern so it can fit anything. Plus it's lighter, smaller, takes less power to run, etc.
Your '78 Turbo Buick would have had the TH350! And turbocharging tends to add to the output of even a puny six cylinder!
If you want to read up on the 200-4R here's a link:
