My advice for switching to a 9 is always the same - buy a housing built to your required dimensions. By the time you hunt up one, get it cut, buy ends and have them welded and then locate the spring perches - you'll have nearly the same money in it. And the bought piece will have better tubes and center than anything you'll fish out of the scrap yard. The just buy axles that fit the housing - Moser, MW, Strange all make affordable 28 and 31 spline units that will put up with a lot of abuse. FWIW, the Mustang has about 320 RWHP and I used a housing made by Currie with the small ends on it. The stock brakes bolted right up and I stuck the stock 28 spline axles out of the 8" back in it. they are on their second year of weekly drag racing with no signs of fatigue.

______________________________________
1967 Falcon 4 door 351C - Owner built, owner abused.
70 Mustang 351C / 06 Ranger, 04 SuperCrew parts hauler