Drum Sizes on a 1973 Scamp?

The 10" front drums use a different spindle than the 9" drums so it is not a simple swap the of the drums and backing plates. They never made a 9" drum with the big bolt pattern.

80% of your stopping power comes from the front brakes and 9" drums are plane not adequate for a car driven on the street in traffic. The car that 8.25 came out of would have had front disk brakes, see if you can get the upper control arms, spindles, caliper brackets, calipers and MC from that car. Use the calipers and MC as cores for rebuilt items from the local parts store and switch to to disk brakes to match those 10 by 2.5" rear drums.