There just might be an original sticker on the engine valve cover with the size shown.
If not, there is an engine code stamped on the pass side front of engine between the cylinder head and the water pump. The letter code is needed to find the size of engine, usually, two or three letters.
A simple way of telling which model of core to get, is to look at the heater core pipes in the firewall on the pass side. If they are close to the engine, then the engine is from the Small Block Chev family. If the pipes are in the middle of the heater, as directly above the subframe mount, then the engine is from the Big Block Chev family.
Rear axle ratios can be decoded from a stamped letter code from the front of the pass axle tube. There might be a stamped metal tag attached to one of the differential cover bolts stating the axle ratio....Maybe.
Everett "OBJECTS IN THE MIRROR APPEAR QUICKLY UPON RAPID DECELERATION"