906 or 915 ?

In your case I'd probably have a set of Stealths prepped and use them. Never run any aftermarket head right out of the box. If you must use stock iron you'll probably have the same money in them as prepped Stealths. Design the engine for whichever heads, compression ratio and fuel you decide to use. Design this baby completely on paper before buying parts and starting building.
That’s definitely a good idea