I ran 1.88 intake valve 72 "J" heads from a 340 on my + .040 273, no problem with shrouding, but I may have relieved the chambers around the intake valve. From the early 273 head racers, 1.84 intake was optimum. I think the shrouding is more head related. I doubt the bore is much of a problem as the "J" heads on the 273 ran very well with a stock 340 cam and valve springs. I never mocked everything up to compare 1.92 intake valves or Magnum heads but I'm thinking they would be a little close for comfort. "920" 273 heads flow better than "302" heads in stock form.