49TT - I did some digging through an old paper copy of a NAPA Brake Parts Catalog. It lists NAPA 9220 as the correct 1-1/4" bore master cylinder for your 6400. The current NAPA Part is listed as NMC M9220 and they want about $290 for one.
Some more web searching shows that the "9220" master cylinder has several providers, most being from Dorman, but all equivalent to the NAPA NMC 9220.
Here are a few "9220" master cylinders that you can order to fill your need, spending as little or as much as you want.
NAPA NMC M9220ebay offered Dorman M9220another ebay offered Dorman M9220Jim Carter ME166The Filling Station FS-241AHope this helps you get further along in your brake restoration.
