Wow I didn't know the oil capacity was so much more. So why wouldn't you just swap for an 8.8 then? That could be done for less than a 100 bucks and a little fun with the welder.

Sorry I didn't post to this earlier. Switching to an 8.8 at this point would involve (for me):
-finding an 8.8
-getting a new rear end mounting kit from MM or Boss Frog
-having my exhaust rerouted

All of the above would come to about the same money I am putting out for a built 7.5 but would leave me with another used O.E.M. differential. By building the 7.5 to spec I'm getting an entirely new differential which fits with my current exhaust and hanger setup, has as close to zero lash as possible, a far superior type of limited slip and my choosing of gear ratio. If I was starting from scratch I'd probably go with the 8.8 from the start although I hear from Mustang IRS people that the aluminum cases have a tendency to flex and throw out your pinion alignment...

I'll start a new thread about this shortly when the project starts transitioning form a pile of parts in a box to something worth writing up.

-Jason