Rebuilt 340 really difficult to turn over

I had the engine rebuilt in September but I'm just now getting this thing in the car. About two months ago I set the timing on it and was able to turn the engine fairly easily with the crank bolt and a breaker bar.

Fast forward to today, I wanted to prime the engine right before I put it in so I went to turn it with the crank bolt and I couldn't get the engine to turn. The bolt felt real soft in there like something was going to strip out or break. I tried another bolt and same thing. So then I decided to just press down on the fan belt and turn the fan. Well that ended up snapping the fan belt and I was only able to turn it 1/4 of the way before I got worn out. The thing was so tight I was lifting one leg of the engine stand off the ground while trying to turn the fan.

Finally I took the engine off the stand with the cherry picker and ran two bolts into the backside of the crank where it connects to the tranny. I used a metal bar to then turn the crank over and it finally started turning, albeit with a ton of pressure. I'm going to need to turn it while it's in the car to line up the flex plate to the flywheel and I feel like I'm going to tear the threads out of the harmonic balancer if I run a couple bolts through it and use a bar to turn the thing over. What do you guys think is going on here? I don't have the spark plugs in. I was able to get it turned a couple times to prime both sides but it never really got any easier to turn over at any point.