Unfortunately, most of the aftermarket mechanical fuel pumps are junk. I have ended up replacing them on every Rover with a simple electric pump. However, the pump cannot be mounted in the engine bay, as it will get too hot (particularly in the summer) and can cause vapor lock (bucking).
Something else to check, if you have your original fuel tank, is for rust in the lines and a clogged pick up tube in the tank itself.
61 II 109" Pickup (Restomod, 350 small block, TR4050)
66 IIA 88" Station Wagon (sold)
66 IIA 109" Pickup (Restomod, 5MGE, R380)
67 IIA 109" NADA Wagon (sold)
88, 2.5TD 110 RHD non-hicap pickup
-I used to know everything there was to know about Land Rovers; then I joined the RN Bulletin Board.