A fairly common mod for the 6 cyl is to machine the carb pad on the intake to accept a standard 2 brl holley carb. The 1 brl carb pad cast into the intake is one of the bigger restrictions to flow and power that the 6 cyl has to deal with, so if you go to the trouble of machining the area to swap on a TBI, you might as well follow the same guidelines used to open up the intake for a 2 brl carb and get the flow enhancements for your effort. If you go that route, it isn't a far stretch to drill the carb pad to take a 2 brl holley pro-jection unit. IIRC, there's even an adapter available from holley to bolt the pro-jection onto a 2 brl intake, that might make the job a little easier. I would avoid using the early GM single injector TBI, they don't have the support or performance potential of the aftermarket TBI systems, so it would be a lot more work adapting one of those GM units to your Ford, with plenty of custom mods to install the system and ending up with less adjustability for tuning and most likely less hp potential compared to the holley TBI.
"It's not having what you want, it's wanting what you've got!" - Sheryl Crow