Should I put a piece of plywood (or something with a little "give")
under the new heater? My concern is that if I don't install the four
12x12" tiles under the heater so they are all the same height, the
weight of the filled 50-gallon heater might crack the highest tile. My
thought is to distribute the weight of the heater more evenly.

I'm not sure if a drip pan is a solution.

Thanks for your advice.

Ray