It's a CFI car. There are 2 fuel pumps on the early fuel injected cars, low pressure in-tank & high pressure under the car. If clearance is needed you can run a '86-up high pressure in-tank pump and eliminate the under car pump. No wiring changes necessary, just use the whole pump assembly from the newer car. Leave the '84 sending unit alone, it gives a different reading than the later unit. However, there should be clearance without fuel pump changes, mock it up and let us know.
Of bigger concern is the rear brake line. Unless the rearend has already been swapped to the later 8.8 with centered body to differential brake line then there will be insufficient clearance between passenger side pipe to rear soft brake line, creating a potential safety hazard of having no rear brakes at some point in the future.
The brake line can very easily be changed, again using factory parts. Just get the hard brake lines from any '86-up Mustang 8,8. They are direct fit to the earlier 7.5 housing and have a center location for the body to diff brake line. Use a new soft line for the later car. Better yet, get a braided stainless line from Maximum Motorsports. Super easy & cheap upgrade, but a necessary one far a safe car, in my opinion. While swapping the hard lines it is a great time to replace the rear wheel cylinders if they have any age on them.
As a fellow 1984 LX owner I'd love to see pictures of your car. There aren't that many of them out there.
Jonathan