305. Even more of a boring pos than a 350. http://www.jalopyjournal.com/forum/showthread.php?t=1494&page=2 Don't pay extra for a number when even the experts can't tell. In our trucks it is hard to see more than the top of the engine.
can't be a '57 283...it's got late '60's valve covers and intake manifold and quadrajet carb...unless you're saying that the block is a '57 283. ...couldn't say what heads those are.
should be a 997 casting number (pull a valve cover to be sure) 1957 head, chamber similar to power pak but has sharp corners in plug shroud area
If dont remember wrong so does all the sbc have the holes for the front mounts. But im not a chevy guy
283 will rev nicely and have torque in your application. I got the High Torque 283 in my 64 Dually. a 327 would be a fine option too. That truck could've come with both from the factory.