[Arran]

The car radios that used the 12 volt "space charge" tubes usually used a transistorized power output stage, they were an odd sort of hybrid, I have a Ford car radio and a few Chrysler ones that are that way. Just a guess but I think that you may be able to get away with using a pair of 43s in place of the 48s, the pin out seems to be the same but the 43 is a pentode whereas the 48 is a tetrode, the suppressor grid is connected internally to the cathode. Also the 48 uses a 30 volt 400 ma filament rather then a 25 volt 300 ma one, but given that the 48 filaments are connected in parallel the current difference isn't an issue just the voltage. The only problem may be that the 43 won't operate as well on 32 volts B+ but it is certainly worth a try as the specs in the tube manual look similar.
As for supplying the 32 volts someone on the alternate forum was using a surplus transformer from a big computer printer I think which may have been a 35 or a 40 volt job.
Regards
Arran