Service Replacement (SR) blocks do not always have a code stamped on the front of the engine. They do, however, usually have a casting number. This casting number is usually on the RH side of the rear portion of the block - where the transmission bolts up. 
Here they are:

400
Year Numbers
1967 9786133
1968 9790071
1968 RAM AIR 9792506
1969 9790071
1969 RAM AIR IV 9792506
1970 9799914
1970 RAM AIR IV 9799915
1970-74 481988
1975 488986 (early) 500557 (late)
1976 500577
1977-79 568577
1978-79 (T/A 6.6) 481988 xx
428
Year Numbers
1967 9786135
1968-69 9792968
455
Year Numbers
1970 9799140
1971 483677 (early) 485428 (late)
1972-74 485428
1973-74 (SD) 490132
1975-76 500813