It's entirely possible the dipstick is the problem. How do you know it goes deep enough, or is the right one? You don't, really.
If it were me, I'd get a long piece of wire, welding rod, etc and check to make sure I could hit the bottom of the pan with it. Then I'd add oil, 1 quart at a time, and notice where it comes up to. Then I'd compare that to the dipstick that I had, and work out whether it was right, etc.
How do I know? I've done it on custom oil pans.
Also, open the drainplug and see if 6 quarts come out. I'm not sure how, but just maybe something got put together wrong. If you don't get close to 6 quarts out, something's wrong somewhere.
Good luck!
George
