340 or 360???

It really all depends. If you find one that is already a bit souped-up but hasn't been thrashed too hard and has good compression and oil pressure then by all means take it. HOWEVER the condition of the bottom end is more important than the HP stuff in the top end; try to find the lowest-mileage, least-abused engine you can whether or not it has any hop-up parts in it.

Oh and by the way the 340 and 360 are small blocks, not big blocks (those are the 383, 400, 440, etc.).