I had a custom aluminum radiator made and all the measurements look ok, except where the fan switch screws into the radiator and then the 2 wires for the fan plug into at.

I told the guy it was 3/4" but I believe he put in a 1" bung.

My question is on the switch, on one side is stamped, 82/77 degrees C
and the other side 2/5

I'm not sure what that translates to or if it is a rating that tells the fans when to come on and off at?

I just need one size bigger with the same train of thought I guess?

Yes, temp range. 3/4 pipe is huge, and 1"is massive. Most of the gm ones I've delt with are 3/8 npt, maybe up to 1/2 npt

Despite putting in the next size up, will I be able to even get a 1" with that range and is the 1" size going to hurt anything as long as I get the temp range correct?

I suppose I could use a brass reducer bushing 1" male to 3/4" female to make it functional again.

You can get aluminum ones and stainless ones to.

Does the switch have 3 terminals? The one from my Audi has a lower temp switch and higher temp switch. The lower temp switch turns the fans on at slow speed and if the radiator gets too hot, the higher temp switch turns on the fans at high speed.

The switch for the Audi is 3/4 inch.