Here is some info on the Ford 9":
http://ultrastang.com/Page.asp?PageID=6

 

Thanks that helped. So what would benefit me, going with the 8 or the 9"? I found a 9" from a 68 mustang and the guy wants $400 if i remember correctly and for a 8" i think he wants $150.

 

The 9" is a 3.00 gear open carrier complete drum to drum with the e brake cables and the 8" is complete with 3.25 gears.

 

get the 9"

 

any specific reason?

 

9" holds more power than the 8", so it'll last longer with your 390.
i believe all mustangs that came with 390 from stock were 9"
further the 9" are getting more and more rare on the scrapyars (from what someone wrote on this forum).
So basically ****** it while you can, as latest when you blow the 8" or want to upgrade anything you will curse yourself for not getting the 9"

 

I agree. The 9" units are getting rare, but the reason for using it is that it is the correct housing for your car and will be an easy installation. The 9" will hold the torque of your FE with no trouble. See if the driveshaft that came out of that car is laying around nearby.

 

Im going to try and get it but ive found another one for $350. I know the 9" is stronger and thats what came with the big block motors i was just wondering if there was any other reason. I thought the 9" rear ends were posi but this one is just a open carrier.

 

Back then most differentials were "conventional" open rears. I'm told dealers were actually told to not push the limited slip rear on 390 cars, to cut down on warranty work, since the 390 could easily spin both tires, spinning only one cut down on potential drive train damage.

All Mustangs with the 351 2V or larger were equipped with a 9" rear from the factory with a 9", all 289HP had the 9", the BOSS 302 had the 31 spline 9", and oddly all J code 68 GT cars had a 9", even though the J code was kinda lame.

 

great info that