I got my P-15 Paramount in January of 1972. I removed the Gran Turismo a year later and replaced it with a Shimano Titlist (basically a Crane with some steel rather than alloy in the parallelogram) and when that wore out a year or two later with a SunTour VGT which remained on the bike until it was stolen in 1991. Both the Titlist and the VGT were dramatically better than the Campagnolo. I did have some contact between the VGT cage and the 32T large sprocket. If there was a B screw I didn't know about it, so I solved the problem with a big file, filing off the edge of the cage that was interfering.

Yes, this was a very common upgrade. It made perfect sense in every respect: lighter weight, much better functioning, and it didn't cost much, either. If "period correct" means bike-in-a-time-capsule exactly as delivered by the bike shop, then keep the boat anchor. If it means set the bike up the way owners in 1972 and 1973 did, then by all means go ahead and put the Sun Tour on, and while you're at it remove the slippery plastic handlebar wrap and replace it with cloth, the way most owners did; and take off the suicide levers, too.