good luck trying to ID the block, i havnt had much luck with mine.
the most for sure way to find out is by pulling the heads and measuring the bore. however, there is only .005" between a 400 and a 455. there are numbers on the passenger bank on the front face and the rear casting below the head.
mabe youll get lucky!
you can also look under the #1 exhaust port for a cubic inch casting.
if it says 350 or 400, it would be a post 73 block(i know in the 70's they started stamping them, just cant remember what year)