I have a ford 31 spline 9" with moser axles, a true-trac locker and disc brakes. I have new cal-tracs and new racing leaf springs to go along with it. Already spent 2k on the rear, 337 on cal-tracs, and 250 on springs.

I also have a 8.8 IRS from a cobra, complete system. the conversion kit for the IRS into my car is $1500+ having to get gears @$160+ new polyurethane bushings.

will the 8.8 handle 690 fwhp, and if not, what will I have to upgrade to make it do so?

 

690 hp on a stock irs is like a ticking bomb. It might take the abuse for a while, but its only a matter of time before it grenades on you. You should getan irs brace as well as upgraded halfshafts. and u can forget about running slicks with the irs

ur 9" moser is a much better choice.