Simplest way I know of to put front disc on a caliper style set of triple trees is to buy a new front disc wheel and make a plate to mount your caliper onto. The threaded disc brake adaptors I have used have the same thread as a standard free wheel. In theory one could remove the free wheel off a rear hub and thread on the disc rotor adaptor. Then flip the wheel and use it on the front with the disc brake on the left side (normal placement) Not theory - done it on the Grape Ape until my new wheel came in. The caliper plate was easy out of 1/8" steel. wraps around the axle and a small bolt holds it in position.