Lots of info on HybridZ.org regarding NA engine builds. Typically if you're starting from scratch:
F54 or N42 block
P90 or N42 head
Flat top pistons
Spend the money on the head. With a P90 you'll need to shave it .080 and shim the cam towers. You can go to 10 to 1 CR on 91 octane with a modified P90. With the N42 you're limited to about 9.5 to 1 CR on 91 octane unleaded unless you weld up part of the combustion chamber. Both heads need work on the exhaust port side, not work really needed on the intake.
BTW... if you're looking for starting cores, I've got a 1975 N42 long block for sale for $250.
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