CNCCustom,
Swertel's rule of thumb is good advice. You also asked about "better" alloys and tempers, by which I guess you mean stronger alloys and tempers. You didn't provide tempers for your 2024 and 6061 alloys, but there are stronger alloys available, 7075 being one of them.
Suggest you use the applied load to determine the strength you need in the joint, then select a combination of alloy, temper, and thread which will do the job. Be careful, also, that drilling and tapping, or other machining in the vicinity of the threaded holes, does not cause local annealing, which can result in a drastic loss of strength.