Oh boy, I know this is frustrating. Has the engine ever been rebuilt? I had one with exactly the issue you are describing. The shop that decked the block not only screwed up the dimensions front to back, but side to side as well. A few thousands of an inch may not seem like much, but when all the tolerances stacked up it made for an intake that would never seal. I had given up on this ever running again. I had a friend look at it and knew right away what was going on. I got another block and got another machine shop. Lay the intake on the heads and check all around with feeler gauges. That will tell the tale.