Recommendation is to use a Ford 8.8 from a Mustang. Here is an article on the considerations from the ffcars forum:
http://www.ffcars.com/forums/120-factory-five-33-hot-rod-forum-sponsored-e-t-wheels/287934-best-donor-car.html
Here is some info that I got from Factory Five about donor rear axles.
Solid Axle Rear End Info
The ideal rear end to use in an Mk4 or 33 Hot Rod kit is a Fox Body width (59 ¼” from hub to hub) 5-lug 8.8 mustang rear end.
The Fox Body Mustang generation ran from 1979-1993. The rear ends from 1979-1985 used a 7.5” ring & pinion and switched to an 8.8” in 1986. The 8.8 is much stronger. All of the Fox Body rear ends came with 4-lug axles and drum brakes stock.
In 1994, the rear end changed to 5-lug and disc brakes. Anti-lock brake sender rings were added to axle shafts, making them 15/16” wider per side, bringing the width out to 61 1/8” from hub to hub. The housings themselves from 1994-1998 remained the same as the earlier ’79-’93 rear ends.
For 1999-2004, the width increased further to 62.52” from hub to hub, this time with both the axles and the housing becoming wider than the previous generation.
Our 14970 Moser rear end is manufactured in the Fox Body width with 5-lug axle shafts. It uses a 2005 generation center section and is thus set up for the 3-link suspension only.
The wheel and tire sizes we spec out are assuming you are using the fox mustang width rear axle. 59 ¼”
Solid Axle Rear End Specs:
* For a 9" wheel, it would be desirable to have a 6" B.S.
79’-93’ 4-lug Rear Axle is 59.25" hub to hub (4.25” 4-lug bolt pattern)
94’-98’ 5-lug Rear Axle is 61.125” hub to hub (4.5” 5-lug bolt pattern)
99’-present Rear Axle is 62.52” hub to hub (4.5” 5-lug bolt pattern)
To run 315/35R-17 tires on an MKIII Roadster it would be desirable to have the 59.25” width rear axle. 10.5” wide wheels with a 6.85” B.S. fit well with the 315s, and are easily attainable in the aftermarket from several of the OEM reproduction wheel suppliers. With custom wheels, use the same backspace, or with an 11” wide wheel, run a 7.1” B.S.
To shorten the 94-98 rear axles to 59 1/4". He said that the 33 has move room inside the fenders so it is not as critical as with a roadster. I think the 94-98 GT 8.8 is the best setup for a 33. It has disk brakes, track-lock and 5 lug axles.
On 7/5/12 purchased a used 8.8 rear from a '07 Mustang GT with 69,000 miles on it.