wat engine do i have?

i have an 81 gmc sierra 3/4 ton that i just pulled out of some bushes. Pulled it into the shop dumped some gas in the 4-barrel carb and it fired right up.. . only problem is, not sure wat size the motor is. Its a v8, original motor was a 400, but not sure how i can tell whether its a 305/350/400 that is in it. I know there is a number on a hard to reach place on the block, but i was wondering if there is any other distinguishing features that anyone knows about that might help me determine what it is. Thanks

You need the casting number off the block. See http://www.mortec.com/castnum.htm

There are links on that page that tell you where to look for the numbers.

//2000CAYukon