400 casting numbers

I'm looking at a 400 block to possibly put in my 73 dart. Wanted some help with the casting numbers. 3698630 400 2. Cast in 1973. Sources say this was cast from 73 to 78 and there is a different cast number 4006530 that is for 76 to 78. Why would mopar have 2 different cast numbers for 76 thru 78. Was the earlier 73 to 78 cast blocks better or cast in a different plant?.