If your intake has a hole in it off to the side of the carburetor that the plate covers (they did this on the 302, can't remember on the 2.8, been too long since I had the carburetor off my dad's '77 Capri) then you can use the 2-bbl EGR plate from any of the 74-79 Mustangs (1979 Fox body Mustangs were also available with the 2.8, pretty rare option, as they switch to an I6 mid year) 1974-1980 Pintos or Bobcats, and the 1976-1977 Ford (Mercury here in the U.S.) Capri, as well as the 1979 Mercury Capri.
As far as where to buy one, check any and all junkyards you can find, look on ebay, use car-part.com, MustangII.net, you'll have to do some digging to find it.
If it is like the 2.3 plate and they used a pipe to run the exhaust to the back of it... Then just ditch the damned thing and use a Mr. Gasket 2-bbl adapter/spacer plate in it's place (I forget the part number, but it's the plate to adapt a Rochester 2-bbl or a Weber 2-bbl to a Holley manifold or vice-versa, it fits perfectly in place of the EGR plate on 2.3L engines when deleting EGR and keeping the stock carburetor. You have to drill out the un-drilled portions of the bolt holes to make the Weber-style carburetor work, but they do work great.) Then you just have to plug the hole in the exhaust manifold with a pipe plug.
(Again, not sure which system Ford used on this one, looking at the picture of the 2.8L manifold in my old Clymer shop manual for the Mustang II it looks like the later, as I don't see and EGR passage in the carburetor mounting plate in their illustration. I don't have my Haynes, Motor, Ford, or Chilton manuals with me at the moment, they're down in the garage with the car.)
__________________ 1976 Ford Mustang II Ghia: 302 with a 600cfm Edelbrock carb, Edelbrock Performer 289 intake, Dynomax Blackjack headers, 2.5" exhaust with Flowmaster Super 44s. RJS 11-gallon fuel cell, C4 tranny, chrome 16" pony wheels, fuzzy dice, brown vinyl half-top, and painted in the tackiest color ever (harvest gold, that's why I call it "The Goldenrod").
Also have a 2003 Dodge Ram (lightly modded daily driver/tow rig/office/dining room/home away from home/workshop... I call it "The Big Blue Dawg".) | 
