Don't take this the wrong way but if you're asking these questions i have to wonder if you've enough experience to be welding on a rollcage
Filler wire would be regular ol' A15. Rough rule of thumb for amperage would be 30-40 amps per millimetre, things like arc length and travel speed make quite a difference. Actual amperage best determined by welding up a practice coupon i.e. knock up a joint or 2 using offcuts, weld it together and cut open so can have a look for fusion defects, lack of/excess penetration etc.
Edit, just re-read the origional post and realised it was 'roll cage tubing' not necessary a roll cage. Appologies/disregard the first sentence if you're looking some pointers/starting point for practice welds etc
however, MSA does state that they are the minimum thicknesses. i think i would feel much more safer in a car of that age with a roll cage of a thicker wall.
just saying.