Just keep in mind, you want the tort to have an option to move to and from heat. So try to keep one end with the ability to be a bit cooler. Think 3 little bears.Great job.
This is the hardest part, staring at an empty enclosure! I just did it myself. Longest month ever! Lol
I'm not surprised that you have low temps in that long of an enclosure. If it were me, I'd set up 3 CHEs. One in the middle, one on each end and then I'd plug all 3 into a single thermostat. This will dispense the heat, evenly over a greater area.
If you had a single heat source, it would have to get much hotter at the source for the heat to reach the far ends. You could end up with 80° corners and a 120° center.