Whole lotta fun here!! As to your roll cage question - looks to me like it's welded to a plate that is, in turn, welded to the floor. Anything on the underside? If it's just to sheet metal, that's a bit light - in my opinion.

At any rate, this will be a great project. Lot's of the hard work has been done.

Have fun and keep the pictures coming!

Merry Christmas
Glenn