More progress made, as the passenger seat bracket holes were drilled (studs were rusted) and the stock brackets look appear to fit perfectly.

However the driver’s side holes, due to some previous patches and brittle metal, aren’t visible. So, I am attempting to estimate their location. The rough distance I measured is from the front of the hat channel bracket just behind where the seat posts rest to the seat brackets shown below. The 16 1/4 inches might be a little short. Anyone have the correct measurement for a circa 1960 FC-170?

Here is the driver’s seat from underneath, per Will’s question:

Here’s an example from another FC-170 with the same brackets (from this post):

Passenger seat: