I've installed two sets so far. I use a small bottle jack to hold the bars up against the floor with jack stands under the rear end and under the frame rails in front of the K-member.

Once I get them to with in 1/4" to 1/3 of an inch, I tack weld them in and massage the floor pan with a big rubber mallet to close up the gaps before welding. I also reversed the polarity on my Mig welder and started the stich welds on the connector and then the floor pan.

Once welded, I sealed the weld with 50 year, paintable roof flashing caulk.