I had occasion to replace the rad on my 4.6L composite recently, due to damage sustained last year when breaking a lower ball joint (see separate posting)
I managed to bag the last Ace radiator in stock at RedLine (a rare article indeed) but did look around at some alternatives. The main problem I found, was that none of the stock replacements seem to have the four welded fixing brackets located at 170mm (6.75") measured down from the top, two each front and rear, with (what are supposed to be) round fixing holes in them.
The closest aftermarket rad I could find was
http://www.carbuildersolutions.com/uk/aluminium-radiator-693mm-x-458mm-cobra-dax compared to the Ace radiator dimensions of 703 (27.75") x 435 (17.125") x 60mm (2.36"). Some of the aftermarket Mustang rads measure up to 95mm (3 3/4") in thickness, which may cause a problem with accommodating the electric cooling fan. However, the Mustang 1993 unit does look pretty close
http://www.cjponyparts.com/flex-a-lite-aluminum-radiator-manual-5-0l-1979-1993/p/RADA14/ I would certainly be interested to know the actual spec of Mustang rad supplied to AC Cars and am guessing the fixing brackets were added in house.