If you built a cage with tolerances large enough to fit your welder in behind to do the welds such as the A pillars or a windshield support bar. It would be horribly in the way visibility wise. The mirror is the least of your concerns, its trying not to light your interior on fire or burn the snot out of your paint. When it comes to cages there really is only one way to do it right, the rest is just garbage. Look into a sanctioned race rule book that is the best place to learn what is acceptable for roll cages. Safety is not a place to skimp!
I've also done a couple cages where I tacked everything together, removed the main B piller hoop, layed the cage down, welded front stuff together, stood cage back up and finished welding B pillar. Normally though, I'd just bore holes in floor and drop cage down to weld on top and on sides.
'80 CJ5 w/AMC360, T176, D44 w/Detroit 4.56, RE 4" YJ lift and 35s..