Rebuilding a 65 273..I need some help.

A 220*cam might power peak at 4800.
1.88s compared to 1.78s? in a 5000rpm engine? IDK if if it's worth it, not to me anyway. But if I did the swap I would for sure notch the bores to eliminate shrouding. Else you may lose more than you gain. Notching is not a hard thing to do; just stay out of the ring-sealing area, and the Fire-ring area. But yeah, IMO, the engine will need to be torn down and thoroughly washed afterwards.
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If you do not yet have heads, consider a pair of Magnums. They already have the big valves in them, and a better port is the bonus. You can run hydros, and adjust the preload with the adjustable arms, to the minimum required to keep them quiet. Jus saying.

Ok. It sounds like more work than it's worth for me. I'll just prep the heads and do a nice valve job and put new springs on them. Thanks for the advise.