Help with engine install...

[TonyzPony]

I could really use some help. I have a 71 Mustang with a 302 that finally kicked after 189,000 miles. Instead of rebuilding it I found a complete 302 with only 45,000 miles on it that had been pulled from a 79 Mustang in a rear wreck for only 500 bucks. I bought it, pulled the old engine, swapped my Edelbrock intake and 4bbl Holley over to the new one as well as changed water pump, oil pump, new gasket set, removed all the smog components and so on. The problem came when I was putting it back in. It was a pain to get the engine lined up again with the converter and tranny but finally got it. Started all the bolts then torqued them down to mate everything up. Tightened the motor mounts and all that was left was to install the nuts from the flex plate to the converter. Got one on then went to turn the crank at the pulley and it was locked solid. wouldn't budge an inch. It rotated freely before while aligned to the converter without the bolts. All mated up I don't know what is going on. I don't think everything would have came together it the converter was off the tranny or was jammed. Before I spend a whole nother day pulling it back out I was wondering if anyone had any thoughts as to what was happening and if I missed anything while installing.