GT40 heads are basically 69 351w heads with pedestal rockers, 1.84/1.54 valves. That's what's on my 13 second Mustang. The P heads have slightly smaller exahust valves and are supposed to make better low end power but the spark plug angle means unique manifolds or headers and I don't know of any that fit other than a fox body. Your LTD might have room enough for fox headers.
A lot of times depending on headers you can run 90* boots but they look ugly and it screws with the routing somewhat.
I have looked into EFI myself. I am more than happy with my Edelbrock 4bbl but I would like the peace of mind for off camber and elevation changes. FITech and Holley are the main two I am rolling between. Summit/Jegs sell the last gen FiTech under their name which is about as far off the beaten path as I would go brandwise.
Howell isn't much cheaper but runs a GM TBI throttle body with their wiring and tune... may have more off the shelf support for parts though rocking GM stuff. You can find a dead or dyin' GM that might have a good injector just about anywhere... not so much with a full aftermarket unit.
That is my big fear, product support down the road.
For a car that won't stray far from home or see much trail time... not sure I would stray from carberation though.