Cisco-
Nothing on the Bronco is ever "DONE", therefore, "everything" needs to be modular-able to be bolted and unbolted (with the exception of bedliner, paint). The cage should be fully welded (no bolts holding IT together) and welded to 4x6 1/4" steel plate. These plates are then bolted through to the floor. Most people go this route...some weld the plates to the floor( mine was done this way by the race car guy that I had make it 12 yrs ago--I didnt know any better at the time. He brought 2X4 tubing off of the frame at an angle, then the main hoop right off of that through the floor. The rear tub mount was welded to plate that was welded to the floor--no frame tie in there. So only the main hoop was tied to the frame).
To take it a step further and make the cage stronger/safer, it would be smart to bring tubing off of the frame towards the cage's floor points and again use 1/4" steel plate as the lower mounting point--effectively sandwiching the body between the cage and cage/frame tie in. Alot of people when doing the cage tie-ins include a poly-bushing set-up somewhere between the frame and where the lower tie-in plate mounts to provide some ability to flex the body.
I always thought this was the only way to go, but when I did my 6-point tie-ins, it ended up not happening. To explain--I thought that the direct frame tie in was what always made my truck ride harsh, but this ended up not being the case. When I replaced the body mounts (WH 1" lift-mount), the ride improved. At the same point, I also cut the roll bar where the tubing went through the floor--I had to b/c of the 1" BL aspect of the new body mounts. I used 3 pieces of 1/4 plate in my case, 1 for the main hoop on top of the floor and 2 (1/2" total) pieces underneath to make up the 1" from the lift. After it was all bolted together it was essentially a rigid set-up, but I noticed no harsh ride, no flex problems. Shortly after the BL and I also had the cage completely redone--already in the plan. The rear section from the hoop back was removed and new tubing was bent, a front section was also added to give a very sturdy family style cage. He also connected the front and rear sections to the frame via 1 5/8 0.134 tubing...he left the main hoop alone (its already very beefy). All of these tie in points use ~4X6 1/4 steel plate under the body and on top and are tied together with 4 pieces each of grade 8 hardware. There are always multiple ways to do anything, but this worked for me and I am very pleased. Hopefully, it will never end up "turtled"...but if it does, my butt is sitting on seats attached to the cage, not the floor--yet another thing to consider. You want to have your seats and harnesses BOTH attached to the either cage or to only the truck....it would be best if seats and harnesses are attached to the cage. If by chance the cage seperates from the rest of the truck you dont get choked or filleted by the seat belt.
Back to your questions, I would recommend not welding the plate to the tub, bolt it down and use 1/4" plate below the floor (as well as the top) to help reinforce the tub...actually a very good idea...then if you ever decide to do a frame tie-in, just connect it to that plate already there. Its up to you about the seats. As with all things Bronco, this is a snowball rolling out of control down a really big hill, only you can decide how big you want it and where to stop. Good luck with your choices.
John