BigDog...
are you sure that the last two stamps are IM? IM is listed as a 69 Impala 350, 250 horse 2bbl with a powerglide

It is more likely an M preceded by the number 1. IF so, the engine is listed as a truck 265 or 283. Since the number before the M is a 1, it is a 283.... a 160 hp 2bbl manual transmission installed in a C10, 20 or 30 ( or a K10, 20, or 30).

mike in tucson