273 for early A

Early 340 cranks are balanced for heavy forged slugs. and heavy rods.

That's a good cam for torque and fuel mileage, but with only 38* of overlap, she won't have hardly any idle-lope to her, which you seem to have alluded to. And log manifolds will kill any that mighta tried to sneak thru.

That cam will probably not want any more than 9.5 compression ratio, which with a ten over 273, makes a minimum total chamber size of 66.5cc. That will be tough I think, with those pistons.
At 8/1 it's gonna be real soft on the bottom and pretty abysmal performance with your listed gears.
As will be said;
Good luck