Well I'll tell you right now - it's one of three things:
1) The head gasket was improperly installed or torqued down, causing a leak somewhere. If the alignment dowels were not placed back upon install, this can cause the head to shift ever so slightly even if it was torqued and bolted down correctly and could cause a leak or tear in the head gasket, or just shift the surface area also causing a leak.
2) The head gasket itself was damaged in the process and cannot hold the seal - and is thus leaking. Could be a combination of head gasket damage or improper install, or one caused by the other.
3) The head or the block (or both) are warped/uneven.
Those are the only reasons for this to be happening. Good luck what finding out what the actual problem is. Head gasket maintenance isn't that hard on these cars, as long as you have like 2-3 days of down time, and that's granted you're not some professional. If you're a pro it can be done in an evening.
Let us know if you need any more help or advice during the process, and what you discover!