273, or 318?

273 said
Bore size definitely not ideal but big enough for more power than most would build with a 273 probably with majority of 340/360, to me rpm be the most limiting factor, since most seem to want peak power in the 5000-6500 rpm range so about 1:1-1.3:1 hp per cid. 270-370 ish HP
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With that smaller bore you can probably get away safely with another 0.5 to 1.0 point more compression without detonation. That 350HP to 370HP should equate to close to 300HP at the rear wheels. For a street cruiser that will get you down the road just fine.