I'm not sure how black widow longbows are cut at the shelf but, if the bow is cut to center or past center, it can still qualify for the longbow class by building out the side plate to 1/8" outside of center.
"LONGBOW (LB) (formally MTRD)
A longbow shot with fingers directly contacting the bowstring using a glove, finger tab, or bare fingers, Archers may use up to two (2) nock locating devices, but they must use the same nocking point and anchor point for each shot and the index finger must contact the string in the same place for each shot. The index finger must touch the nock of the arrow. . Face and/or string walking are not permitted.
No sighting device of any kind may be used. There shall be no markings on the bow or bowstring (intentional or accidental) that could be construed as sighting marks. No type of draw check or clicker may be used. Arrows shall be made of wood and shall be identical in length, weight, and color (allowances shall be made for normal wear). Arrows shall be shot off the hand or shelf of the riser. LB archers will shoot from the white stake. A longbow shot in LB shall conform to the following specifications:
1) The bowstring, when the bow is strung, may only contact the nocks of the bow.
2) The riser should be cut no closer than 1/8 inch to the outside of the centerline of the bow, however, if a riser is cut less than 1/8 inch to the outside of the centerline of the bow, a pad may be added so that the arrow rests on a surface no closer than 1/8 inch outside the centerline of bow.
3) No stabilizers, counterbalances, or weights of any kind may be attached or built into the bow; except a bow quiver clearly designed to hold arrows.
4) A longbow may have a draw weight of up to 90 pounds (measured at the face edge of the bow at the arrow shelf @ 28” draw)."
http://www.ibo.net/Rules/