Do you have a picture of the engine pad? The engine is actually out of a 66 Impala. The carburetor designation (the R in IJR) was only used in 1966. Additionally, the N is Norwood assembly, which stopped producing fullsize cars after 1966.
I could not find the block casting.....on the back of the block HI PERF is cast on the passenger side. There is also a date cast toward the front of the block, October of 1966 if I interpret it correctly