What compression are you running?

You can get away with anything lower than 11:1 on pump (supreme)gas. Just don't lug the engine.

408stroker5.0 said:

You can get away with anything lower than 11:1 on pump gas. Just don't lug the engine.

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Probably with an aluminum head and premium you could, but not on an iron headed with 87 octane.
9.7:1 is plenty low to continue using 87. I might go up to 10.3-10.5:1 for pump gas, but I would be listening carefully for detonation and hoping I didn't have to retard the timing a bunch.
A tiny bit of extra compression isn't going to give you that much of a noticible gain in the real world. Error on the side of caution.
Can you mill the head much on a 5.0? If so, you could mill it down a little instead of getting different pistons.

Notice I put Supreme Gas not Regular Gas which is 87 Octane. You could not run 11:1 on 87 octane.
Supreme which is 91 Octane is usually enough but a little octane boost would be a good idea.

I run 11.25:1 with aluminum heads and super. Have yet to drive it on the street, but ran fine on the dyno, first test drive hopefully this weekend
mike.