I just measured a 1956 block. The bolt hole depth is 1.25" and the threads stop around 7/8" in into the hole. The head (with gasket) is right at 4 inches thick. As I recall, the head bolts are something like 4 17/32" long, so it is pretty unlikely in my mind that you could be hitting the bottom of anything...if this helps you feel better about that...

~ Jon
1952 1/2 ton with 1959 235 | T5 with 3.07 rear end