Jerry,
Good luck. Probably a lot more 261 engines around than '53 235 powerglide engines. You might check and see if the bosses on the side of the engine are drilled and tapped for the side engine mounts used in the '53 Chevy car. If not drilled and tapped then chances are good that it is a 261.
I'm not an expert on this like pre68dave and others here, but I do have a '53 powerglide engine which is how I know about it.
Kurt
