Ok, I am back to searching for parts for the locost project. And am looking for a rear diff. I would like to know about the RX7 I am looking for disc brakes, LSD, and a face to face distance of about 42-44 inch as I seem to recall.
So what junk yard years models should I be looking for?
85 GXL-SE?
I could very well be wrong. Please take my reply with less than a grain of salt.
Lolz! Here's the run down:
79-80 (SA Series1) only had drum brake rear ends.
81-83 (FB S2) you are looking for a GSL (like mine). 12A, 4x110 bolt pattern, rear disc, LSD.
84-85 (FB S3) you can continue to use the GSL for 4x110 or you have the (expensive) option of a rear out of a 13B-powered GSL-SE, which is also rear disc and LSD, but with a much easier 4x114.3 bolt pattern.
86+ (FC) use IRS.
I don't know what the face-to-face distance is, but I'd honestly be surprised if it was that wide. Of course then you could run some ridiculous dished wheels! 
Strongest is the 84/85 GSL, has a 3.9 ratio twin clutches on either side and has disc brakes.
GSL-SE has extended axles which bring the offset from the GSL's -28 to -40 in line with most of the FWD offsets, but some say are weaker due to the added length. They also have a strange 3 on one side then 1 on the other set up for the LSD clutches which is also credited with being weaker, they have 4.1 ratio and bigger discs.
Early GSLs have smaller bearings and axles and are definately weaker.
Cars with GSL rears have been known to run 8s 1/4 times but this is not personally known by me, I am running a GSL rear in my 5.0 FB and a GSL-SE rear in my 350 SBC FB.
Sorry to bust your rear axle length bubble, but 1st gen RX7 axles are about 53" long face to face. They vary a bit with disc vs drum brakes. I know this because I have one getting ready to take up residence under my latest project.
aeronca65t posted a link to a really cool site where a guy showed how he narrowed a GSL rear to go under a Spridget.
They're seriously that wide? The whole car doesn't look 53" wide! 
Yep, they are that wide. Rear track width is listed in most references as 55.1 inches. The front is 55.9.
(resists urge to make bad joke about mis-measurement of male member)
~HERE~ is the narrowed RX7 rear that JM referred to.
It required 12" total removed (6" per side) to be used on a Spridget.
Uhmm....I have all those parts in my garage.. http://picasaweb.google.com/jessemorehouse/GarageCleanout
zip is 49428
been trying to sell them.
full IRS rear end with the TII LSD unit
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