Mark, my closest 'guess' based on a X4 is a XA = 77 model 400 V8 180 hp motor,
or other guesses are a (XR 73 model 350 V8 2 bbl 175 hp), , XN 73 400 4 bbl 230 hp, (X7 , XM , XE 455 250 hp, XR 350 71 model 250 hp), 71 XX 400 265 hp, 70 XP 400 370 hp, XX 400 265 hp,but all those are from a Firebird book, emngine could also be out of another Pontiac... but my best guess is a XA since its so close to X4....??
all this is just the block no...