I finally got them out; was a real hassle. I used 409 to loosen them, then using a small hydraulic floor jack, applied pressure under the seat base - between the flange in the floor. Then, used a hammer to strike the base plate on the opposite side of the jack. This simultaneous pressure allowed the base to come out, slowly!
Thanks for your input.
Edited by DFW-fisherman (07/23/17 09:43 AM)