Welcome to the forum
Interesting story on your first '65 Chevy truck.
I think there is a comparison thread here somewhere. You might look in the '60-'66 FAQ index at the top of the '60-'66 forum main page. Anyway here is a quick summary. The '60-'63's have the wrap around windshield, The '60-'62's have the torsion bar front suspension, the X-member frame and a hydraulic clutch. The '60 & '61's have the bugeye or snorkel hoods. The '63-'66 have coil springs in the front and a ladder type frame, the same as trucks thru '87. The most common differential gear ratio in a C10 is 3.73. In the C20's it is 4.57. It is not difficult to change the differential in a C20, or to make it a C10.
Good luck with your search. Just a FYI, there is a trucks for sale section on this forum. You might find something close by.