Why did I spin a main bearing?

yeah its a 4 speed, when i got it home i checked everything with the cam, valve springs, lifters and oil pump i primed it and everything looked good. so i made the mistake of trying to start it again. oddly enough the starter cranked it fine but its a high torque mini starter, only thing that clears my headers. when it fired it took about half throttle to hold it at 400-500 rpm so i shut her down right away.

So when you primed it--What did the guage show? You are not providing enough info. I will say that if you needed half throttle to keep it running--You are looking at something major. J.Rob