I'm sure a lot of these heavy trucks exceeded 60 downhill back when they were the only thing haulin freight cross country - how fast it can spin isn't the right question, how long it'll last doin it is what you should wonder - have you replaced the rear axle bearings? even inspected them? looked at the condition of the gears? .... and as Tim points out, how it'll handle at 60+ is another question, the front ends weren't designed for it ..... your rear gears will be 5.43 at best, quite possibly 6.17, using your tire size and gear ratios you should be able to figure RPMs
Bill
