as long as the head-barrel is Ok, it can only be rings or valves. I would put the head back on without pushrods etc and see if you have compression, if not you may be able to hear where it is escaping from. A leakdown tester is brilliant for these problems!
Possibly the tappet clearances were not 100%?
Anyway, good luck and let us know how you get on.
cheers
Pete