Upgrading tank /heater questions

[Ilzakor81]

I’m planning to go from a 10 to a 29 gallon tank, I currently have a 6x17 ultratherm and am planning to get the 6x23 when I get the new tank, is it a good idea to keep the 6x17 and use it on the side even though it will be hanging off a few inches?

[I<3mycrabbies]

The smaller heating pad should fit on the side if you put it vertically
If the dimensions of the tank is 30 long 12 deep and 18 high