You can verify the year by looking on a machined pad on the left side on the engine. It is on the block towards the fron just under the head where a large letter E will be cast into the head.
The numbers stamped on the machined pad should read something like 30MXXXXXX
3 = Oldsmobile
0 = 1970
XXXXXX = the last 6 of the VIN of the car it came out of.
455's will be stamped with 8-6 for the second number. Production was from 1968 - 1976.
There will aslo be a set of numbers cast into the block on a shelf just behind the water pump nest to the oil fill tube.
They will be 396021 F.
Good luck with the sale.