How do you determine "ride height" to make suspension mods??

I was figuring that the rear rims where going to have to be widened to 12" which I was thinking on having a set of the Bullet rims stretched to the 12" (yes added cost, I know but at this point I have tossed the budget way out the window, LOL)

Hmmm... now to find a set of used rims and tires on CL in 18" (I guess ford pattern) and as close to a 335 as possible.... I will have to see what that will cost versus just buying the rims and tires now....

Having Weldcraft or similar company widen the rims will work. The mock ups are really for the front, since that is much more difficult to determine what will work. The rear is really just a tape measure number with a straight edge