1970 340 has the 1973 low compression pistons

Depending on your expectations and gear ratio. 340s with flat tops with a zero deck and iron heads is just about all you can get away with on pump gas. With those Pistons you will be okay, but for around $300 and a set of kb243s you'll have a stronger setup. But if you're looking to put on a torque storm or something like that you're just right.