1968 912 porsche?
 

It's a Mb Star C4 912. Leave it as a 912 and enjoy it for what it is.

If you are dead set for horsepower, and are going to butcher it anyway, then put a small block Chevrolet V8 in it. You'll have a rocket ship. It won't really be a Porsche, but it'll fly. 300hp is nothing for a SBC. Standard crates make 350hp and up. I have a Healey with a Chevy V8 that makes over 440hp on the dyno custom built for me by a NASCAR engine builder in NC. If you get serious email me and I'll give you their contact info Mb Star Connect C4.

You'll need a 915 transmission (or a G50) to handle the extra power, but you'd need that anyway to handle your stated horsepower goal.

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