I have both a 1-ton GMC (two of them actually) and a 2-ton Chevy and though the Chevy looks much better in the pictures, I'd go with the 1-ton if you want to build a highway capable regular driver. You'll have to make some changes, but not as many as it will take with the 2-ton.
But the better you can define your intended usage and performance requirements the easier it is to make this kind of decision.
The motor looks like a '54 or later Chevy and would not be the original in either of those trucks. Its hard to be sure from that photo, but it sure looks to me like a four cap screw type valve cover instead of a two nuts type. And I see no sign of a GMC logo embossed in the top. So I conclude '54 to '62 235 or 261.
Parts are easier and cheaper if its a Chevy motor, but the GMC motors are better in a number of other ways.
