GMC sixes don't lie. There is a flat horizontal boss just behind the distributor with a serial number punched onto it. The first 3 numbers give you the size, most likely a 270 or 302, but could have been changed for a 261 chebby motor. It is also possible, but not probable given the size of your beast that the motor is one of the larger gmc sixes that got as big as 503 cubic inches.
