the UTH on the side of the tank is not that good of an idea. it's not going to do much but heat up the glass. bumping the ambient temps up is best done with a lamp.

what is wrong with your temps now, too warm or too cool?

the larger UTH will def help the belly temp, but make sure you have a thermostat or at least a rheostat cause you don't want your snake to get a belly tan.