Leak could be due to a warped head...did you check it with a straightedge before you installed the head?
Otherwise a faulty head gasket, nick in the mating surface, or leftover material on the mating surface could be culprit.
I see two ways for fixing this problem:
- Go to the auto parts store and get "Great Stuff" and seal the outside area of the head where it's leaking. I see people that make LS/VTEC motors do this since the B16 head leaks oil externally near the upper radiator hose connection all the time.
- Pull the head off get the head and block checked for being true and replace the gasket again.
Either way, I'd get it done ASAP as motor oil on the timing belt wears it out extremely fast.