Disc brakes don't necessarily reduce the overall width. Rotors are either mounted to or are part of a hub which has bearings that are supported on the spindle. The wheel is subsequently mounted to the outside of the hub. To determine the overall width you would need to measure the outside dimension of the hubs on the current axle and the proposed replacement. There can be a lot of variables depending on fender clearances, wheel width, and offset.

Fred.


1956 3100 Pickup/Red/350/3sp OD/PS/Disc Brakes
1957 Bel Air Sport Coupe/Red/355/TH350/PS/Disc Brakes
2017 Silverado LT Single Cab SB/Black/5.3/6 Speed Trans
1947 Willys CJ2A w/F-Head engine