F059 WS Block code 9790071 is RAIII 400.
Use either block, both are good, just use the one you need with the motor mount bolt pattern for your application. Holes can be drilled and tapped in the 70 block if they're not already done.
Raising compression with the 7.6 CR 5C heads would require more work than 16's, and Since you don't mind buying pistons, a custom dish with the 16s could drop you into a good pump gas friendly power making range. As Jim said, the 16s could stand some work. But even as is, they were 290 HP with 2bbl, with 4bbl and duals they're closer to the 330 HP range. Work the 16's and pick the right cam, your HP wishes should come easy.
Run your numbers again, they're all good at Wallace Racing!
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