I've owned several Z's in the last 15 years, and the only nice one I got was from Cali. Which is the one I still own. The East Coast is very hard to find a good car. The South West is going to be the best area.
Even when you do mange to find a nice rust free car you will still need to stiffen up the chassis, if you want to put any kind of power under the hood. And before you do that I would make sure that the Chassis is straight to begin with. 30+ years of driving will have twisted the chassis a bit. It may not be much of a flex, but why stiffen up the chassis if it's already twisted.
My nice clean Cali car (With 0 rust, or visable signs of an acident) is in 3/8th of an inch on the rear pass side strut support, and the pass side of the car is 1/4 inch longer than the drivers side. I took these mesurements myself so they're probably not exactly right. I'll no more when it gets onto a frame machine.