That axel might work for you just fine. I'm not sure about the width though.

The problem you might run into is the difference in backspacing between the original rims and modern rims. The original rims had the mounting surface in the middle of the 6" rim, so they had 3 inches of backspacing. Most of the modern rims I looked that are 6" wide have the mounting surface towards the outside of the rim and had 4" of backspacing. So, your new tires will be 1" closer to the frame, which might work or not.

If you want to use wider tires you have to use wider rims, which will also mean a wider axel or custom rims. Standard steel 7" GM rims I think have 5" of backspacing.

I tried a 62" axel in my '51 truck with 6" rims and found it was too narrow so I went with a 65" and 7" rims that is just right.

A 2 1/2" axel tube is kinda narrow, most axels are 3". What Truck '26 says might be on the money, even if the width and torque specs work, the weight capacity might not be what you need. Maybe you can get some info from GM or another source about the capacity of those rear diffs. If they were run of the mill corporate 10 bolts, they're probably okay.

'51 Chevy 1/2 ton w/'62 261, HEI, offy, fentons, dual carter/webbers, t-5 & 12 bolt posi