Both cars have drum brakes. (power discs standard on COPO) It's unlikely they are anything more than cars that have had a big block added later on... The big block heater core was likely added for the same reason GM did it, because a small block one doesn't fit too well and mashes the heater hoses between the firewall and cylinder head if its not changed. Yeah, the discs could have been removed for a full drum brake setup, but thats usually very unlikely. I've seen heater cores swapped out to accomodate a big block swap alot more often than I've seen a full power disc setup scrapped in favor of 4 wheel drums. Anyone ever drive/stop a 69 Camaro or chevelle with 4 wheel manual drum brakes? It aint fun.
