Yes a 750 is way too big for most applications. A 750 would be more suited for a 440 and some people even use them on BB strokers!

If I were you, I'd go with a 600 for stock/mild 340 or maybe a 650 IF the 340 is a pretty wild. Sounds like a 600 would be fine for your application.

Overcarbing will make the motor a slug down low...