Have you tubbed out the rear? If not, you're going to have a hell of a time fitting a 10" rim back there! Those 66-67 chargers are very tight back there in stock form. I don't think anything bigger than a 275/60/15 on a 18" rim would fit. Even that is a squeeze.

You don't want your front tires sticking too far out or they will grab the fender lip when you hit a bump during a turn. They do need to be tucked in a little.

You can play with this calculator and plug in some various tire sizes and rim specs. Just bump the offset box up or down until you get the backspacing to match the rims you're looking at. Then you can pull out a tape measure and see what's realistic.

http://www.rimsntires.com/specs.jsp