Many times there are already holes there that aren't used. Look at your firewall for rubber plugs. Often these are factory punchouts that are there for the quick and easy addition of "options".
The beauty is, you can use the same rubber plug! just carefully cut holes for your lines to pass thru it snugly, so it seals on the line.
My firewall has been plugged (welded) by somone ever the years. There is one large hole, but it has been pop rivited. I will weld it when I tear the truck apart.
I had to drill one hole to get my sending units through. Then used a black rubber seal and then sealed around the tube/wire with black silicone. I do not want any leaks (even a few drops) getting into the cab.
Here is where they are installed. No holes in the dash face. They are mounted underneath on brackets.
What guages you going with? Pods? Yes everyone is right, there are holes, just take the plug out, drill the size you need then put it back in. I already had power wires and a hole from my old lightning driving lights (bless their souls) lol so i just used those wires for what i needed power to. Trans temp senders aint big, you can use a 14 guage wire for that and a O connecter. Good luck buddy if you need help on what i used to get power, switch for lights, and whatnot just holler il see what i can do!