I found a threaded fork to match my frame, but it is ridiculously long (240mm) whereas I really only need 155mm or so (54cm frame).

Scuttlebut is IF there's enough steerer tube left after lopping off the threaded portion, there's no issue installing this as a threadless fork. Thus, if fork steerer is 240mm and I need only about 215 mm worth of steerer (at MAXIMUM), then is only a question of if the threads extend much past the 25 mm cut off point. I estimate about 3mm worth of thread could remain after cutting to correct length. Is that still too much thread to clamp a threadless stem to (using a shim, of course)?