Aluminum frame alloy question

Assume repair/modification of an aluminum sportbike frame (in this case a 1989 GSXR750). What standard aluminum alloys will be most appropriate?

6061 i think but not 100% positive Look up "old school gixer" or something like that i lost my bookmarks.

I've got a few engine parts kicking around if needed.

I would assume 6061 and Pray to god it isn't 7075.

What do you mean by repair/modification? You do know that the frames are heat treated after welding in order to releave stress and build strength correct? If this is a structural piece I would be carefull

I have a project that will require extending the frame to fit a physically larger engine. That will require inserting plugs to the frame tubes OR welding two frames together. That's what I mean.

Yes, I know the frames are stress-relieved.