we have a mindset especially here in north america that the engine should be in front. rather than placing it where it might make more sense. think of the original VW beatle people wanted to look in front for the engine it was what their mind told them was logical. the closer you can place engine to the drivewheels the better, now in the case of a large gas engine you end up some distance away. however the steam motors that would function in a pickup are going to be roughly 18 by 24 or less so mounting it either behind where the present transmission is, or right on the diff, lets you eliminate the weight of the drive shafts and all the space they take up. in the case of the old stanley i believe the engine, firebox, and boiler were all one. if i understand correctly it didn't really have a boiler where water was stored as it made steam on demand. which is why the time to make pressure was a few minutes rather than 30-45 minutes. boilers radiate a great deal of heat, as well as any piping between the boiler and any engine, that heat loss greatly reduces any economy. the stanley brothers eliminated much of it by making it all one component, and was a great part of why they could almost compete with gas engines. even those torpedos basically used the same steam on demand concept, they needed instant steam with 0 wait time