significant changes that I have made on my 1954 Suburban, in order of value & performance & enjoyable drive-ability are:
2000: 3.55 R&P conversion (and ALL new bearings/bushing/seals - venders sell kits - double your costs and spend the money)
I can't believe the crazy price that the big vendors are demanding for the bearing & seal installation kit.
Just like Steve did.....You can locate all of the associated parts needed for the 3.55 conversion on your own for a fraction of the price.
Deve has documented the individual part #'s for the installation kit in his article on the 3.55 gear conversion.....
More info.
Here