Either rack will fit. Any tbird, cougar, mustang, capri, fairmont, etc will fit the car. TRW or Ford.
However... There are two types of rack input shafts, round ones with splines, and round ones with splines that have a flat on them to index to the intermediate shaft coupler. You have to match that characteristic, more than anything else. The inner tie rod ends for a tbird are longer, but can be used if the outers are threaded a bit deeper, or just change to the mustang inners.
The hose inlet fittings also vary, but most racks come with an assortment of adapters.
I think the Ford rack housing has a one piece housing, while the TRW has a two piece main housing and a separately removeable control valve housing. (or vice versa) The Ford racks will have Ford cast on the main rack housing, and I think you can see it with the rack in the car, even.