I have a 1946-7 gmc water truck from which I removed the engine from and the serial number doesn't match any I can find, it reads n169573cal, and yes this is the number off the plate adjacent to the distributor, would anyone be able to help me out?

If this number is on the flat behind the distributer your engine is not a GMC. The first three digits should be 228, 248, or 270. Those indicate the displacement of a GMC engine. I bet someone switched in a Chevy 6 in place of the GMC. Look at the cast numbers on the side of the block and try to match them to the ones listed on this site.

The numbers N.....Cal are numbers assigned by the DMV. I think you may have a military engine.

it is most definitely a gmc motor, and that is intriguing, would that mean it would be a 302

Check the block casting numbers against the lists on the website 'Casting #s' section, also sometimes there is a brass tag rivited on the lower left side of the block if it had been rebuilt. It was a common practice in the past for commercial rebuilders to grind the original serial number off the pad and restamp their own catalog number, or leave it blank for the DMV to assign a number.

Tim