I was going to suggest Reynolds 531, it would probably be lighter as well.
This is an interesting and recurring theme. With over 1000 M3Ws out there, someone must be working on a self-built chassis.
Not sure Reynolds 531 is the way to go - it is high ductility (for butting) rather than the highest tensile grade for plain gauge tubing.
Makes me wonder what it would cost to have a frame made in Reynolds 531 Manganese/Molybdenum alloy steel. It is light and very strong.
True, but Reynolds 531 is not intrinsically that much lighter than other grades of steel - its superiority comes from the fact that it is ductile and suitable for double-butted and triple-butted designs. Car tubular chassis' typically use straight-gauge tubing - but no reason why a butted design couldn't be used. Caterham built a proof of concept - at not inconsiderable cost!
http://www.pistonheads.com/gassing/topic.asp?t=1550657 
