A method for equipping an engine with a mixing device for introducing gaseous fuel into an intake passage of the engine comprising the steps of providing a body through which the intake passage extends, wherein an annular channel encircles the intake passage, providing a fuel inlet manifold with injector receiving ports, each injector receiving port configured to receive a discharge end of a fuel injector, with each injector receiving port in fluid communication with the annular channel, and providing a fuel supply manifold including fuel injector ports, wherein each fuel injector port is configured to receive an upstream end of a fuel injector. The method further includes blocking at least one injector receiving port such that fluid communication through the injector receiving port is prevented, removably coupling fuel injectors between the fuel supply manifold and the unblocked injector receiving ports, and providing the mixing device to the engine.