A little research into the "air bearings" which really should be called air casters.
Air Caster Floor Specifications and Floor Requirements for SOLVING Air Bearing material handling equipment. Floors for Air Casters
For these to work properly, the inside of the tube, after welding, will need to be honed. 2% of the diameter of the bearing. Since the bearing is 3.5' x 4', the effective diameter is A=pi(d^2)/4 or d=sqrt(A*4/pi) = 4.22. Honing will need to be to 0.085" on an 88" tube. That is pretty darn close tolerance. This is +/- 0.1%. Metering tubes for measuring flow accurately are +/-0.25% for just the first pipe diameter.
An alternative to metal honing would be coating the inside with an epoxy compound and then honing that. Again, extreme tolerance are required to make the air caster work.
http://www2.emersonprocess.com/siteadmincenter/PM%20Daniel%20Documents/Meter-Tubes-DS.pdf
0.005" inches.