I am going to attach my seats to the cage.What size tube should I use to support the seats? And how did you attach your seat belts to the cage?

1.5 OD to run from the middle spreaders to the front spreaders, 1 OD bent around the transmission tunnel to the 1.5 OD tube you just used. Put your seats on there and either make tabs or use angle. Seatbelt is attached by tabs.

I used 1.5 inch tubing .120 wall.