I had this problem too. I had my 427 on the stand for over a year and a half, inside, assembled, oiled bagged , etc. I turned it every few weeks with no problems.

I went to put the engine in, put on the flywheel, oil pump, pan etc. I got the engine in and went to prime the oiling system, went to rotate the assembly it turned about 1/8 turn with a great deal of difficulty and refused to turn any farther. Bam it seemed locked up.

I didn't turn the engine directly prior to putting it in... WTF... Ok, so I started at the oil pan / pump, crank wasn't hitting anything... Ok... Hmmm... well was the ckutch hitting the bell housing / flywheel... Maybe... Ok out came the engine...

Nope. Bellhousing wasn't the problem... What else could it be... I decided to put it on the stand so I could turn it upside down, was something lodged in the crank??? I unbolted the clutch, then the flywheel. As I got the third flywheel bolt out the engine turned!

It seems the ARP flywheel bolts were longer than the Ford units. The cluprit was the 427's balance included a few hundredths being removed from the bolt face of the flywheel, thus allowing a number of the new bolts to hit the back of the block... I shaved a few hundredths off each of the bolts, weighing each to maintain a neutral balance on the bolts and all was well...

Hope this helps,