Hy stiffy, the GMC engine is a little longer than the Chevrolet engine, but they are swapable. The engines share the same bellhousing mount so you can set the rear of the engine in the exact same place as the Chevy, and you'll only have to adapt to the front mount differences. The GMC engine should have a serial number stamped into the pad to the rear of the distributor, the number you found might indicate the engine is a 236, in any case the first three digits of an American GMC engine indicate its size. For example 228, 236, 248, 256, 270, and 302, hope that helps.
i was on here a while ago asking about a 37 chevrolet fire truck with a blown engine. i think it's a 216, but i'm not sure. in any casy i have acquired a gmc 6 in good shape, but i don't know if the motors are compable. the gmc was not in a vehicle so i have no vin #. the only #s i can find are on a plate on the left side. 23624295. and another set of #s 2185849. is there any way i can determine the year and size of this motor. thanks.