My understanding is that there is no hidden number on the early trucks. If someone can suggest where to look I have a bare frame out in the yard I can easily check.
If you can post some pictures of the truck there are lots of possible clues to its actual year.
A '50 would not have wing windows. A '51 would have wing windows and turn-down door handles, whiel a '52 has wing windows and push button door handles. Of course doors might be swapped, so its not conclusive.
A 292 is from a different family of motors, and uses a bellhousing the same as a Chevy V-8. Your current bellhousing will accept the 235/261 Chevy motors and the 248/270/302 GMC motors.
If you use a '54 transmission crossmember you can mount a '63 - '67 bellhousing that will take the 230/250/292 six or any Chevy V-8. The newer six requires some of the same changes a V-8 will require, but does not require messing with the steering, which by it self is an attractive advantage.
