~450hp from stock stroke 340?

Aluminum heads are absorbing compression because they’re a heat sink. If your static is 10.5:1 your cam choice should be in the 9.5-10:1 range.
In theory but in practice I don't believe there's a power difference between the two but yes seems you can squeeze a bit more cr from aluminum heads and run on pump gas probably less chance of hot spots.
The crate motors made power because the came equipped with cast iron heads flowing 25 more cfm than the J head untouched.
I don't know about 25 cfm more, Charles Servedio flowed both and are fairly close to each other on intake side, J had slightly more peak flow and magnum had slightly more low flow, exhaust magnum did a lot better, probably that and more do to port and chamber shape for hp gain.