The measurement you need for your rear is flange to flange. Thats the outside surface of the brake drum where the wheel mounts, not the inside.
My guess is that your measurement will come out to around 58-60 inches.
Don't try to us the stock rearend. Those cars were designed for speeds much less than you'll drive on the interstate and if you're going to pull something it won't stand the stress. Once you have your measurements there are plenty of web sites that can supply you with the width and year of rear ends that will fit your car. Then all you have to do is pick one that meets your needs. Highway driver with mild hp (300 or less) Ford 8" or similar. More than 300 hp and the occational trip to the strip, GM 10 bolt.
Big hp, GM 12 bolt, Ford 9" or similar. The Ford 8" and GM 10 bolt are easy to find and if you don't need a posi unit are cheap too.
<a href="http://www.hotrodsandhemis.com" target="_blank">http://www.hotrodsandhemis.com</A>
[ July 19, 2002: Message edited by: Centerline ]</p>