[dtw]

Oh dear...is this the engine I think it is? Did I hear something about loose perimeter nuts on this one?

I'd detach the chains and back off all the rockers or just pull them, and try turning the cams by hand. If you're lucky, the cams are binding and you can investigate from there. If they are binding, loosen the cam tower hardware and knock the towers loose from the heads with a mallet and see if that frees up the cams. If so, maybe a simple re-torque (after re-sealing the towers to the heads) might work. "Lucky" being a comparative term as this will involve machine work if the head and cam towers are not flat and a re-torque doesn't solve the problem.

If you're not so lucky and the cams turn freely in the torqued cam carriers, but the engine is still not turning freely, then crank is probably binding. After you've disengaged the cams from the crank, you can turn the crank and see if it is hard to turn. If so, the case might have been torqued incorrectly. More likely is that the case was twisted a few thousandths but the builder did not have the main bearing bores cleaned up - now the new bearings are binding up the crank. If this is the problem, you've got a lot of work to do. The case would need to be measured out and cleaned up, and if it was me, I'd get a brand new set of main bearings. Then you'll have to clean and reassemble it all again. I guess one other possibility is that the crank isn't straight.

Let me know if you need some help with this in-person - not sure where you stand on engine work DIY but I'm happy to swing by some weekend.