crank should have been keyed when blower went on...wayyyy too much tension for the LS setup. I have no clue why they didn't key them from the factory. Especially on a motor base that is making such good numbers STOCK. I'd try to bolt it up, remove bolt, tack weld pulley to crank (if possible) and reinstall bolt while you save for a permanent fix.
I did something similar on my old 79z28, broke the key, ended up using a helicoil on the main bolt (which a few racing forums recommend even on new cranks because they are stronger). It help up fine after that.