360,340 or 5.7 Hemi?

I would use the 360 you have. A 340 won't give you much (if anything) over the 360 except the ability to say "Yes it is a 340".

Single 4 vs. Six-Pack......

The single 4 (tuned correctly) will give solid, trouble free performance and deliver the same power as a Six-pack.

The Six-Pack has a WOW! factor that can't be matched by anything short of a Cross Ram. You will have to spend a bit more time tuning and is 4x the cost.

A 5.7 Hemi requires SIGNIFICANT work and extra cost. Do a search, several members have or are doing the 5.7 Hemi swap.