will dp 90 work for a finish coat on the bottom of floor pans or does it need to be painted. If so what is a good flat black. thanks in advance

That is an excellant choice. Does not need to be top coated.

I did the frame and underneath of my '69 3 years ago with just the DP90. It has held up very well.

Sheetmetal, DP90 would be fine as a top coat on the floorboards. But as putting it over bare metal I would not do. I would first put on a wash primer like PPG dx1791 then apply your DP90. The dx1791 has a much better adhesion and corrosion resistance characteristcs then dp90.