Vibration in steering wheel would point to potential in suspension components between the wheel and spindle.
Vibration from seat/floor points to different component. This is where items like driveshaft etc come in.
Something is either loose or out of round.
What is run-out of the tires when the front end is off the ground?
With one tire off the ground can it be turned to get any lateral or turning movement (as if steering)?
If run-out is below 0.070" on your steel wheels and since you have changed tires it is not the tire uniformity so must be some other component or the centering of the tire on the wheel or wheel on hub.
Suggest you review
this article in detail.
pg 42 of
THISmanual may help.