Help choose engine for my 68 Barracuda

Put the 360 crank in the 340!

THIS!!! ^^^^^^^^^^^

I never got that one, true a larger engine can make more power at any given rpm but given similarly built engines wouldn't the 340 would just make it at slightly higher rpm?

Yes

Since you already picked your gear the largest engine will work best. The cheapest would be a 360 or a 3.58" stroked 340.

Yes! I would favor a 3.58 stroke crank in a 340 and a over bore for 372 cubes.
(Then fully ported Edelbrocks and a small solid roller, 1-3/4 headers and ........)

But WTF do I know?