How about taking some pictures of the motor. Is it a 351w, 351C, 302, or ???
351w has 7 bolts to hold the valve cover on and the 351C has 8. Also, the 302 and 351w block has the intake bolts going at a 90 degree angle to the block...meaning the bolts are pointing straight towards the ground.
The number you posted doesn't mean anything to me....can you find the block casting number like DZ, CO, with numbers and letters and should be just above the starter. It should read something like D1ZE-6015-BA.
If you Mustang is a 73, they made no R code engines for the 73. If you Mustang was a 72 then yes, there could have been an R code motor in it....although R codes would be a 429....if the vin is H then the car came from the factory with a 177 hp 351-2 barrel. If the vin was a Q it would have been a 351-4 barrel.
If the motor is actually a Q code it could be a 351-CJ and could have 4 bolt mains which would make the motor quite valuable. We need to know the Ford casting number before we can make anymore educated guesses.