MSD distributor trouble...

Any advice out there? I'm re-installing my intake manifold and went to put the MSD Pro Billet dist back in proper place but it will not seat properly all the way down. I've tried moving several teeth back and forth, turned the crank/cam a couple of degrees back and forth and checked down the dist hole with a fiber optic light to check for obstructions and no deal!
The engine is a Chevy 383 Stroker with solid cam and aforementioned dist which removed easily. Anything special about this set-up that I need to know? Thanks for any help!!

Thanks everyone, but I got it. It finally dropped into position but I had to "re-sequence" the firing order.