cheap heads-free 340 heads on a 360

depends on the valve sizing of both heads
if the heads on the car right now are 2.02 exhaust then I wouldn't even touch them
if the heads on the shelve are the smaller ones (what is it, 1.60?) then I wouldn't even touch them

even if the ones on the shelve are the larger ones you should consider what work might have been done to the heads that are on the car right now
if the heads are ported, blended, maybe even milled down a bit then there is a good chance you will loose power by swapping