Your 450 would either have a 16600 or 1700 Eaton. They were very good rears and Eaton built them for many years. Parts are still available. All truck manufacturers that offered a two speed used them. I always found more Eaton parts at Ford dealers than anywhere else. International dealers also stocked a lot of parts for them. Even while they were offered, GMC didn't stock many parts for Eaton rears. The main thing that goes wrong with them is the shift box. There is a pair of limiting switches that corrode over and there is a small ball bearing that corrodes if water gets in the box. But these parts are still readily available. Here is a parts catalogue
