Heavier? I am assuming this IS NOT a skinny wheel road bike or you wouldn't be adding all this junk. With big heavy shocks on the front end and (I assume) a great disk on the rear, you are worrying about weight ;-?
Another thought - it is possible to determine approximate strengths of heat treated aluminum by hardness testing. See the URL below for additional details (note that is all one long URL)
You might be able to determine, very closely, what you have by hardness testing.
Another thought - Sail boats often use 6061-T6 for spars and I doubt that a 50 ft. mast is re-heat treated after welding.
A final thought - Why go it alone? All of the bike magazines have question and answer columns. Why not write and see whether they have any idea about the feasibility of welding on a T6 frame? After all; guys here are into welding. Guys at the bike magazine are into bicycles...