The earlier blocks are said to have thicker cylinder walls. I'm not sure which years they started and ended this.
Your 72 block may be one of the ones with the thicker walls (they used the 340 cores that were .040" thicker walls).
The 77 heads will work fine. just tap the egr holes for a set screw and loctite them closed (with the set screw below the machined exhaust face surface). then throw in a set of 2.02" intake valves and a little clean up porting, and you're ready to go....
