Bloom, I will have to remeasure it tomorrow when I go down there. If memory serves me right it is 60"...or maybe 62"...Damn CRS. When people talk about rear end width they are talking from axle flange to axle flange. The only reason we went with the 9" is because a buddy of mine gave us the center section. Originally we were going with a 12 bolt because we were going to have to buy a complete unit. I don't know what you are using for power, but I have heard of people using an S-10 rearend and it is almost a direct bolt in.
Here's a great article on installing the firewall.
http://www.webrodder.com/article.php...6&SID=60&CID=7
I found it more helpful than the instructions that came with the kit.
Any more question that I can help you with let me know.
Bernie