When I bought my F150 in 1989 it came with bench seats so I had them
give me two loose top-of-the-line car bucket seats as part of the
deal. Then I sold the bench seats and installed the buckets. No
particular problems - just sat them loosely in place and made packing
between the mounting holes and the floor and bolted them into place.
That was 18 years ago and they have not moved (exept up and down and
backwards and forwards when I want them to).

I shoulda thoughta that!