claudiusx types faster than I do.
Lots of variables in play here. Is the 75gal all glass, or a box w/ glass/plexi doors in front, or what?
I'm in the process of moving a baby from a 16.5gal open-top plastic tub to a 150gal wood enclosure w/ glass doors in front. The same 80W halogen has been able to heat both (moving it from the tub to the big enclosure).
Not much heat will be coming from the UVB, and that subject is a whole 'nother ball-o-worms.
For heat, you need to know where you're starting. Start w/ the basking lamp--it's the most important for heat. I'd move the basking lamp into the 75gal, turn it on for a day, and measure your temps (
temp gun preferred). Once you know that, we can talk about changes/modifications.
One of the biggest issues will likely be heat loss/insulation, which is (inexpensively) in your control.
Depending on the type of enclosure (all glass or glass front) you can insulate the sides (aside from the front) and top (if open/screened) with rigid foam insulation--pink, white, or blue--which can be prettied up if desired. The top can be similarly insulated as long as you leave sufficient openings for lamps (not too close) and ventilation. Measure temps again.
Depending on your basking lamp wattage (you didn't specify--halogen preferred over LED; LEDs don't put out enough heat) you can increase that a step (20-25 watts) at a time
within the safety limits on the fixture and measure temps again.
Those mods
should be enough--even w/o the CHE & heat pad.