Here are a couple of things that would have been original to your model that you may want to consider for the swap.
Common 5 speeds in C60-C65's were Clark 282 and 285 models. Rear ends are determined by capacity and basically what I remember is that the 15K w/ 2 speed was a vacuum operated Chevy rearend. The 17500, 21K were Eaton electric operated 2 speeds. I doubt you'll find any 21K's in C60's but a ton of them had 17500's. Very common farm trucks with 7K front axle's, 9:00 rubber and 6:50/7:17 rear ratio. Hope this is of some value to you.
ps...you didn't say what your truck now has but the assumption is that it is a single speed rear, (?) The model and ratio may still be visable on the inside of the glove box door.
