10 inch Brake Backing Plates - Simple Source?

Can't you get one from any B or E body?

Yes. And from any of these. I highlighted the ones that run the latest, ie, easiest to find in the yards. This isn't a comprehensive list, but it's what shows up on RockAuto as compatible.

10x2.5" drums with BBP

CHRYSLER CORDOBA 1975-1983
CHRYSLER IMPERIAL 1981
CHRYSLER LEBARON 1977-1981
CHRYSLER NEW YORKER 1979
CHRYSLER NEWPORT 1979-1980
CHRYSLER TOWN & COUNTRY 1977-1981
DODGE A100 1964-1970
DODGE A108 VAN 1967-1970
DODGE ASPEN 1976-1980
DODGE B100 1971-1980
DODGE B150 1981-1989
DODGE B200 1971-1980
DODGE B250 1981-1989
DODGE CHALLENGER 1970-1974
DODGE CHARGER 1966-1974
DODGE CORONET 1965-1974
DODGE CUSTOM 1965
DODGE D100 PICKUP 1970-1987
DODGE D150 PICKUP 1977-1987

DODGE DART 1973-1976
DODGE DIPLOMAT 1977-1989
DODGE MIRADA 1980-1983
DODGE RAMCHARGER 1978-1983
PLYMOUTH BARRACUDA 1970-1974
PLYMOUTH BELVEDERE 1965-1970
PLYMOUTH BELVEDERE II 1965-1967
PLYMOUTH CUDA 1970-1974
PLYMOUTH DUSTER 1973-1976
PLYMOUTH GTX 1967-1971
PLYMOUTH PB100 1975-1980
PLYMOUTH PB100 VAN 1974
PLYMOUTH PB150 1981-1983
PLYMOUTH PB200 1975-1980
PLYMOUTH PB200 VAN 1974
PLYMOUTH PB250 1981
PLYMOUTH ROADRUNNER 1968-1975
PLYMOUTH SATELLITE 1965-1974
PLYMOUTH SCAMP 1973-1976
PLYMOUTH TRAILDUSTER 1978-1981
PLYMOUTH VALIANT 1973-1976
PLYMOUTH VOLARE 1976-1980