Yeah, I think you would be much better off putting the lining in first, then remove your seats etc. and try and get the cage in with as little in-car welding as poss. Any yo do do, put a heat shield between cage and lining.
Better off if the headlining is in first and the cage is in bits.
where the hoops are going cut holes in the floor just big enough for the diameter of the tube, car on stands you can then drop the hoop through the holes which will give you plenty of room above to do the welding without damaging the headlining, heatshield on the headlining though as Clive says
Will need a lot of trial fit etc to get the holes just big enough to allow you room to get it back up when the legs are on.
You can then either patch the holes or just cover them with the floor plates
We can do a bar which is the hoop, crossbar, rear legs, side bars and floor plates in bits for DIY fitment, this would need to be worked a little as everything depends on the height of the driver so it can't be just a weld together deal.
The side tubes are notched at 90* so the top is close enough but the bottom depends on where the drivers shoulder and elbow comes so the lower part of the joint will need final grinding to suit.
Last one we got was around £350/£375 from the states