From the numbers you've listed, this is probably a crate engine.
So, here's the skinny on it;
1M0520 - Engine Assembly Code - 1 = Chevy, M = GM of Mexico, 0520 - date code, May 5.
035266 - Possibly a Partial Engine VIN Code, but not for a GM car or truck, digits are missing at the beginning. Difficult to decode.
A Chevy Partial Engine VIN Code would look like; C8A122052 - C = Chevy, 8 = 1978, A= either Lansing or Atlanta, GA (assembly plant), 122052 = to match the Vehicle VIN.
It's stamped on the front of the engine, usually on the passenger side, just below the head, normally behind the alternator.
Also missing from your engine is the Vehicle Assembly Code, usually stamped below the Engine Assembly Code on the passenger, front of the engine, just below the head (see above), it would look like; V1028TWZ; V = Flint, 10 = October, 28 = 28th day (of October), TWZ is the Engine Suffix Code = 1978, 4 bbl, 350 cid, 165 hp, K10 - 25 (K is the designation for a 4wheel drive vehicle, Pickup, Blazer, or Suburban).
The Engine Suffix Code of this number would give us the vehicle it was installed in, but the engine apparently was not assembled in a vehicle, in a GM assembly plant.
cts-908, 13-05-04, 8:40 - written in a paint marker. - Probably an assembly or testing mark from an inspector or assembler where the engine was installed.
Without the Casting Number (not listed in your post), it seems the history of this engine that makes the most sense is; crate motor from Mexico, shipped to a boat manufacturer, probably in the U.S., then built into a larger stern drive or inboard boat.
But, with a cast GM crank and 4 bolt mains, a guy can really build a lot of HP and Torque into this engine as is,if the lower end is tight.
I hope some of this helps!
Good luck with your project!
