As a former mechanic when I wasn't working for a dealership, I always used Felpro. Also, I have never had any problems with the full gasket kits That I used when rebuilding Pontiac v-8's. It has been a while since I did one and maybe things have changed in the last 8 years or so. You said it had an air leak, did you confirm by testing which cylinder it was? also you need to check the manifold with a straight edge and feeler gauges as it might be warped from overheating by either a clogged carb running lean or a past cooling system problem. Also make sure the angles match properly because if the heads were cut enough the intake needs to be cut also. Also, whoever installed the first one may not have used the proper tightening sequence and also did not go back for a second retorque.