Try Grand Canyon Harley Davidson which is located near Bellemont, a little ways West of Williams. Visit www.grandcanyonhd.com or call 928-774-3896.
Just a logistical note is that Seligman is a short distance away from Williams and there's much more of Route 66 that can be explored in this area. You might consider taking Route 66 through Peach Springs, Hackberry, Valentine, etc. all the way to Kingman then take Interstate 40 back to Williams. You might consider stopping at the Grand Canyon Caverns just before Peach Springs - they have a nice little walking tour, that doesn't cost too much $$.
Another thing to consider too is that Williams is about 6,500' above sea level and as such, March/April is considered a transitional period between winter and spring. The weather could be sunny and cool (i.e., great riding weather) or blizzardy and blustery (i.e., terrible riding weather), so start monitoring the weather about 2 weeks before you get set to travel.