Those early Carb Spacers do not fit on the Large Log Heads correctly and you would have a major Vacuum Leak trying to fit it on. In order to use one of them you would need to fashion up a 1/4 inch adapter plate to match the head be able to bolt on the Carb Plate the hole parts not so hard as it can be tapered to match up. Can you post some pictures of what you have. As far as the Cable Throttle Linkage that could be fixed to work right. And if you have a 250 Six in your 68 Mustang then the engine mounts probably need some adjustments to their Height so that you have the proper Hood clearance you need. The 250's can fit into the early cars but getting the engine mounting right is the first thing that needs to be done right.
Like the above Cable a 1975 to 1977 Ford Maverick or Comet 250 Six OEM Throttle Cable Bracket will bolt right onto your engine. When I used to do these Cable Throttle Swaps into earlier model Cars and trucks I would get all the Factory parts the Gas Pedal, the Cable, the engine Bracket, even the few bolts they used. Then install them into an early model Ford it's quick and so easy it's as if they were designed for them. Your Cater YF Electric Choke Wire Plugs into the Alternators Stator, and you need to have a Hot Air Stove Pipe from the Choke Housing down to that Hole in the Exhaust Manifold where the old Tube has been broken off.
