I spend a fair amount of time trying to talk people down from using a 255lph fuel pumps. Reasons being most engines won't need that much flow (unless the engine combo is really producing 540+ HP) and due to back pressure and fuel aeration issues resulting from trying to stuff 255lph back into the tank via the restrictive 1/4" OD return port on the OEM style Ford EFI Pump Hanger. This has been well documented by Chris at Pro-M Racing.
I have come to realize some people just want a big pump. Others already have already purchased a 255lph pump so for them it is too late to right-size.
For those that have 255lph fuel pumps we have developed a high flow 06AN return kit.
It is Breeze P/N 70750. It can be installed on any Mustang 5.0 fuel tank. The price is $43.00
Here is an installed picture: (sorry it is so big)

It consists of a smooth positionable elbow to o-ring bulkhead fitting on the outside, and a special bulkhead adapter and discharge hose inside the tank to deposit the returning fuel at the bottom of the baffle.
To install it you drop the tank, remove the pump and filler neck, empty the tank, drill a 9/16" hole about 2" in front of the fuel pump hole, clean up the chips, install the Breeze 70750 low profile smooth bend 06AN fitting outside with 3/8" ID hose outlet on the inside, pinch off your 1/4" pump return tube, Replace the pump and filler neck, change your return fitting to 06AN Female and re-install your tank.
Picture of the un-installed kit parts to follow.
Any Questions?
Mark Reynolds
Breeze Automotive
508 533 6455