Hey Cylinder heads guys- advice needed

Perhaps, but I have to believe MP sold piles of their 5.28/5.29 length valves for those heads through the years........ and that people successfully ran them.
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However, taking the whole head situation into account, I’m skeptical about how the 273 rockers are going to end up working out on the exhaust.

“If it were me”, I think I’d call HS and ask them how much longer w2 rockers are compared to rockers for stock heads.
I’d also ask how long the rockers are that they make for Mancini.

There is currently a set of 8 new MP 1.5 W2 exhaust rockers on eBag.