68 coupe and convert suspension

[Gmac454]

i am looking to go 17 or 18 inch wheels on my 68 coupe and my convert. they are stock small block cars, with the exception of disc brakes on the front and multi-leaf rear's. what is the best way to find out what tires and rims will clear the fenders and frame with out buying up front and then finding that you are hosed. I am planning on re-doing the suspension like jer67convert but the thought of spending a ton of dough to find out i dropped the front end too much for the rims to fit isn't a happy thought.
thanks