Helm Seat Replacement ??

[2ndCurse]

I finally got sick of the bruises on the back of my legs from the orginal helm seat. I received my new Todd single leaning post today. I've got a couple of questions now that it's time for he swap. How is the original seat post mounted? I had the 1st bolt come right out but the rest just spun. Will I need to cut in a hatch? Are the nuts on the bolts spinning? or is there a plate under there? Thanks