we did an underfloor master/booster set up on my cousins model A roadster pu and as dad said, it was a BIG pain. I'm guessing the Desoto has twice the room under there as a model A has. we used a "universal" kit, which just means you get the master/booster and a generic "fitzall" type of bracket/pedal assembly. this was chosen because we knew going in that most any bracketry would have to be modified to fit as there were no underfloor kits to fit his unknown mfg [likely home grown] chassis. most of the kits out there use a gm[corvette ?] master with a small diameter booster. I really don't know what the booster is patterned after, but for his light weight ride, works pretty good. we also had to re do the pedal as there is really NO room in a model A for anything ! not trying to discougage you in any way, but trying to let you know what you may be in for. our reason for this was to keep it off the firewall both inside[underdash] and out.