See if you can find some stock early 440 pistons and rods to swing in if you really want to up the compression, I just sold a set an hour ago.
With that cam you won't be losing much with those heads, sounds like a good combo other than the gears. Headers will do much more than any kind of head swap to a later set of heads.
I had a road test of a '65 426 Street Wedge car that ran 15s with the lo po cam, log manifolds, 516 heads, and small AFB. Headers, 3.91s, and a recurved distributor had it in the high 13s. Just step up the gear when you can.