a float or coil over shock will work, the beauty of the float is the instant air pressure adustment instead of trying several shock springs. True, this system was not built to handle the shock mount also supporting the weight of the sled, but in mtn sled conversions I've seen, the overbuilt cat arms etc will handle the coil over springs no problem, normal use. Young guys that must launch off all obstacles onto flat hard trail will break the mounts, but all systems under same abuse will suffer. So far not a lot of float lenghts to choose from, Arvins which should work equally well, also limited slection.
A longer shcok can be shortened an inch or two and usually still supply sufficeint travel for the system to work, so there are some options.
