5x4" Drum to Disc Brake conversion

Why don't you want to use the existing drums? Why don't you just reuse the front hubs with new drums? All you have to do is use a hole saw to remove the swedge around the studs and replace the drums. Or find a pair of original front drums still within spec. Rock auto has both front and rear drums in stock. Or find a set of Kelsey-Hays original brakes and rebuild them. You know either will work with your rims.