Tom,
The original rearend in his truck has free floating axles - I believe it came stock with 5.13 ring and pinion.
I have several old trucks like this and have found that a 67..72 3/4 ton HO72 center differential section is a 2 hour bolt-in swap and retains the original housing - thus no need to modify leaf spring mounts, shocks, etc.
If rear brakes are a problem - perhaps the easiest (and if you drive it a lot, the cheapest for maintenance) is to convert to disk brakes . . . which have the added benefit of capturing the axle if the nut / bearing fails
