340 crank in 318?

If everything works well 13-15psi is a higher amount for a street/strip V8. But, if you're not going crazy on the heads and intake the it's not a big deal. You'll just be pushing it hard through a small hole and it will limit power anyway. You should plan to replace the main caps with billet steel and install main studs too. And I'd have the bores sonic tested before you get too deep into it. HIgh horsepower is doable - it makes sense to make sure the block can handle it.
Balancing should be easy depending on the parts. Mallory is normally needed if the rods and pistons are heavier than what the crank counterweights are designed for. If you're running heavier H beams and heavier forged pistons then Mallory might be needed. Your machinist can tell you once you both have all the parts chosen.