6k on the stock bottom end is pushing things. 550 lift will probably be ok. I know you can install PI cams in a non-PI motor to get a perf boost. From what I've been told modular motors can stand up to .600 lift without PTV interference but you'll want to check with an engine builder who's got more time inside the 4.6 valvetrain than me. VT engines is a good place to call if you can get someone to answer the phone.
Springs are a definite, you'll need a tune BIGTIME too. It'll run like a sleepy retard till you tune it properly on the dyno and won't be streetable. It's really too much cam for you.
My $.02... The 268 or 270 would be a better choice, they reach peak power at a lower rpm and are much more streetable. Still has a killer idle, and easily winds to 6K but you don't have to get that high in the band to feel the boost. All you're doing is moving the rev limit further out, and moving the power band out to meet it.
At high rpm with capable cams you have less to worry about up top than the possibility of tossing a rod and venting the block.
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