Would having the engine lying down help that much? Wiki mentions the R-975 having diameter of 114 cm and a length of 109 cm. Add in the fan required to keep it cool, and some kind of 90 degree bevel gearing to move the power from the crankshaft to the transmission shaft, and looks like you end up at an even higher installation?
Radials were just not a good option for tanks. Of course, you gotta work with what you have available, so.. Well, the US could have gone with the Liberty engine just like the British.