Aloha Dan,
That engine # is for a 72 TR6. I had a 72 for awhile and like Ray, was told the '227 head was better because it had the wide intake ports for the better intake manifold yet retained the bigger (32mm) exhaust valves. Who knows if it's true in reality though.
BTW, the head number for a TR250 (and up to 71 or so TR6) is 516323. I bet there's a TR250 engine that will turn up someday for you to grab, rebuild, and use. You are right - the engine you have now will move your 250 down the road and sound GREAT doing it!
Jeff
74.5 & 75 TR6