Donor #1: 1979 Mazda RX7
I purchased a seized 1979 RX7 donor in November of '01 for only $50. It cost twice as much to have it transported home over 30 miles on a flatbed truck, but it was still a pretty good deal. I removed the parts that I plan to use, sold a bunch of parts to recover my cost, and then paid $20 to have the shell taken away (total cost: $170). I actually gained $60 in the process along with all of the running gear (wheels, brakes, axle, front struts, etc), a great pedal box, steering column, and various items like seatbelts and a radiator.
The RX7 live axle
When laying out a custom chassis, the very first thing I consider is the width between the tires. This requires having the axle, wheels and tires all bolted together. The differential has little bearing on the width of the chassis, but it can significantly impact the design of the tranny tunnel. The 1st gen RX7 rear end is no exception. The differential input flange (not the housing per se, just the driveshaft flange) is offset to the right between 1 and 1 1/8 inches.
The tire-to-tire measure of a 1st gen RX7 live axle with stock wheels and tires is about 47.5 inches. When using book style rear brackets, I recommend a custom chassis width of 44 inches to fit the 1st gen RX7 axle... technically this is a "+2" design. My custom build width is 45 inches because I'm using rod ends on the trailing arms and I can afford an ever so slightly wider chassis. A book width chassis will have plenty of clearance but the tranny tunnel may require some modifications to accomodate the differential flange offset, and you will probably need wider rear wings (the CMC 11" version should work well).
The tranny tunnel is the real trick with the RX7 live axle. You can either make it wider overall (to compensate for the 1" offset) or you can offset the tranny tunnel itself (wider driver's seat than passenger's). Another trick that some builders are using is to place 1/8" sheet steel along the left side of the tranny tunnel and remove the 3/4" tubing. I see no problem with this as many designers have analyzed the chassis structure and concluded that the tranny tunnels 3/4" tubing is structurally redundent anyway. Their findings suggest that you could make the entire tranny tunnel out of sheet steel and have a lighter frame with comparable strength, but I've not done any such analysis myself.
Donor #2: 1997 Ford Contour and 199? Mustang
I found a running 1997 Ford Contour that was issued a certificate of distruction by the State of Florida and had just arrived at a local salvage yard. Once I fit a battery under the hood, it started right up and I ran the car for 30 minutes before making a decision to buy everything I might need under the condition that I could pull most of the parts myself. I pulled the computer, fuel pump, speedometer, steering assembly, all of the wiring from the engine bay, and nearly all of the wiring from behind the dash (including the interior fusebox). I won't need all the wiring, but at least it will be easier to figure out where everything was intended to go.
The Contour had an automatic tranmission, but I negotiated with the junkyard owner for a T5 transmission from a 2.3L Mustang along with the flywheel/clutch assembly from a manual Contour. Joining the Zetec engine to the Mustang tranny is not a simple bolt up, but it is possible with some elbow grease and a TIG welder. I have since gone back to the junkyard to look at the old Contour bellhousing (for cross reference) and the owner gave me an identical bellhousing for free! Now I can compare the two on my workbench and maybe even TIG something together with a little bit from each.