Funny you should ask that, I just built my 347 shortblock today. A stock 302 block will be strong enough if you don't get to crazy with it. There's alot more weight swinging around in there, and it's swinging alot farther to boot! I HIGHLY recommend a stud girdle. In fact I wouldn't build a stroker without one.
Block need notched at the bottom of the cylinder bore to clear the rod bolts, so does the girdle. For both the 331 and the 347. You would not believe HOW CLOSE the crank throws are to the bottom of the pistons!!!! Also, now that I've had one apart, DO NOT buy stroker pistons that have the wrist pin down in the oil control ringland. Could give a poop about burning oil, but that situation causes some serious problems that I just found out about today. Stay away from those.
331 vs. 347? I'd go with the 331 given a choice. Less weight/arm/strain on that block that was only supposed to have 255 hp from the factory. 331's absolutely SCREAM, and we have one making 550 (flywheel) ponies, although it's in an SVO block.
Good luck, and let us know how it goes!
