No the factory 360 crank does Not fit into a 340, and
if it did, it would punch the pistons up out of the bores by half of the stroke difference.
The rod castings are the same put the 340 has floating pins versus pressed pins for the 360.
And that pretty much makes the balance question moot; but again, the 340 forged cranks were balanced for the factory 745? gram pistons versus IDK 505? grams for the 360 pistons, so no, a rebalance would be required.
I wouldn't put any non-stock engine together without balancing it. I hate doing things twice.
But
many years ago, Chrysler sold 3.58 stroke cranks that dropped right into any 3.315 stroke saddles, 3.79 cranks too.
A 3.79 in a 3.94 bore/318 comes to about 370 cubes, about the perfect size for an A-body. I bet I could suck close to 28 mpgs out of one of those by building it to cruise down at around 65-1800 rpm. It would, of course, need either a manual trans with overdrive, or an A500/518
