Hey guys,
I can't take all the credit for this I got a lot of help wrapping my head around how to do this from this thread.
I started by cutting the square floor around the mounts out of the donor trying to stay as close to the perimeter as possible. On the outer edges I had to cut inward more due to the rail underneath.
I made stencils for each mount out of cardboard and traced them onto my cargo van cut the slot out to drop the mounts into and painted the raw edges for a little rust protection.
The mounts from the donor were pretty rusty so I cleaned them up pretty good and sprayed rust killer primer on the bottom of them.
I cut the extra floor material around the edges of the mount plate to have flush edges to weld.
I placed each mount in its spot and clipped the seat in and it all lined up. I cut the holes a little bigger for the mounts to have a little wiggle room to adjust them.
I lined them all up and tack welded the corners. I popped the seat out and then clipped it back in to make sure things were lined up and then I tacked a bunch of spots around the mount to stop the floor from warping and alternated sides filling in the spots between.
Next step is to get it ready for paint.