I am not familiar with the 391F material, but at a company I used to work for, we welded 6061-T6 aluminum by the mile. It is a very weldable alloy. As far as strength goes however, in the T6 alloy (the T6 stands for the heat treatement, which happens to be solution heat treated and stabilized) the weakest part of the metal is not the filler (4043 is a recommended filler for 6061-T6), but the heat affected zone of the base metal. The heat from welding significantly softens the base metal. The question that would be most important to me in your situation, is was the contractor using a qualified aluminum welding procedure. AWS has an aluminum welding code (D1.2) that anybody purchasing welded aluminum structures should specify that contractors must qualify welding procedures to before the project even goes out for bidding. Unless of course it is covered by another code, such as ASME, ABS, etc.