What is the largest size wheel that will fit on a challenger and what back spacing do I need for the front and rear. I plan to go with a wider wheel in the back

I assume you mean an early [70-74] model. 'Cudas and Challengers should be pretty much the same. I run 17x8s with 5 in. backspace. I think 18s should fit Ok as well. I think you should be safe with 9 in. width and 5 1/2 in. backspace. You might get away with a little more backspace on the front. You MIGHT be able to even get away with 5 3/4 in. at the rear with low profile tires, but that would be pushing it.