The tire size will affect your ride height more than wheel size. For example, for most of these fat fendered cars people might use a 24-26" diameter tire on front and a 27-29" tire on the rear depending on desired ride height, clearance, etc. Here's a tire size chart that might help you with tire dimensions - Tire Size Chart

As for which Heidts to order, I suspect the crossmembers all mount the same in the frame and to get lower you use drop spindles. I think they also now offer a bolt in kit but you may have to do some frame rail repair depending on how the Corvair suspension was installed.