Want disc brakes, what to do?

A body with front drums got SBP throughout the build years. 8 1/4 cars were disc brake cars.

The cheapest swap would be to find a set of spindles/calipers from an FMJ body car. It does make the front BBP. The wheel choices for you in SBP are limited and the brake systems with SBP are either stock KH brakes or some of the aftermarket stuff.