Mike,
I found the GC engine code to be a '71 6-292 from a 1/2 ton truck.
To verify the info, look for a date code on the side of the block, passenger side, towards the back, between the oil pan and the side cover.
The 0 in the engine code is a zero. The 0413 is the month and the day of the engine (April, 13th).
Bill
