The Blue Ox Base Plate Kit # BX1619 (no longer available) that you referenced will work perfectly on your 1991 GMC S-15 Jimmy (as long as it's the 4WD model) and to connect to your existing Demco Kwik-Tow Tow Bar you would just need to add the Tow Bar Adapter # DM9523041.
The adapter pieces will let you connect the Demco Tow Bar to the Blue Ox Base Plate arms without issue. view full answer...