Welding up the seams fully requires the lower arms be removed, which will require dropping the mufflers and back out of the way, and it makes it much easier if the rear is removed to weld the upper mounts.
As far as inside goes you will need to remove the bac seat, pull the carpet back and be very carefully when welding the seam sealer doesn't catch fire.
Almost all the metal under the car is galvanized, do not breathe it in. If you do not know how to weld it will be instresting for you, and it will splatter a lot.
For installation of a re enforcement kit you can install them with the lower control arms in place, the uppers need to be removed from the body side. Check the bushings in the rear end housing while you are at it, easier to change while it is all apart.
I install one of my lower kits first to make sure the box is straight, then weld the seams...if you weld them with the boxes crooked it will cause issues.