Quick way to figure out volume of a tank on the go, measure dimensions in inches:
(length x width x height) / 231 = gallons
Also, I agree 100% with cristancanoe. With the tank not properly supported, it's possible for the tank to crack, and you'd end up with 75 gallons of water all over the floor! A thick piece of plywood cut to size put on top of the desk will give it some better support.
The heater may or may not work for that size of tank, depending on what the average room temperature is. If the room temp is usually in the low to mid 70's then it should be fine. Just make sure you monitor the temps so if you need to you can get a bigger heater.
Also, as Kemul mentioned, you should probably cut a hole in the mesh where the light sits so more UVB light can pass though.
I bet your little guy is loving the upgrade!
