We all try to do repairs as economically as possible, so you are not alone there.
Some cracks are small or in places you would not see easily. Pressure testing, or magnaziglo inspection (can't be done on aluminum) is the only way some of these hairline cracks can be found.
Then there is the gaskets you used that could also be the issue. I suggest you do a search on the subject here. I have read someplace that some brands are better than others. I know that on my 454 it blew out a couple before I installed a different brand. In that case I had had them resurfaced, tested, and valves redone so I knew they were ok and that left the gaskets as the problem. Since you said you checked for warping and torqued it according to the proper sequence / spec, to me that leaves two possibilities bad gaskets, or a crack. So your problem can be simple, but the only way to be sure is to test the head.
my 2 cents
