TruckDown is a POI file, not a map of course. You can use TruckDown on a 255 but it won't give you anything other than the truckie-specific categories shown in [Only registered and activated users can see links. ].
To get all truck related features you must use a dezl or nuvi 465 ['truckie's nuvi']. nuvi 255W hwid 0827 can convert to 465 but not either chip version of 255.
PS: Welcome to GPSPower!