Steve, if you can, check the height of the valves when they are closed, see if any are higher than the others.
The ol carbed 460s were bad for burning out the valve seats, those that the seats machined right into the cast iron part of the head & then induction hardened only in that area.
The new model FI engines suppose to have hardened valve seat inserts, but you still could maybe have a burned seat gone bad.
When this happens,(receding valve seat)as its called, the valve recedes up into the head & then you'll loose valve clearance, but in the case of the 460, theres no valve adjustments at the rockers & the hydraulic lifter is the only thing allowing the valve to seat.
Once the valve recedes enough, in some cases it will be enough that a valve or valves can not close completely.
So check the height on those valve stems & see if any seats may be burned out.
The ol carbed 460 GBM I had, I finely had to have carbide valve seats installed on the exhaust side. Once done, didnt have any valve problems.
Two other things, since the 460 has no rocket arm adjusments for the vavles, you can buy some different thickness shim to put under the rocker arm post to give that lifter a little relief. Summit Racing has those shims in case you might need some.
If you need some good indepth info on a 460 big block Ford engine, go to http://www.460ford.com now them boys over there love a 460 & got a lot to tell about them.
Neil