Here is an instructional film of the construction of a Piper J3 Cub during WWII...

Part of it illustrates the fixturing and welding of the Chrome Moly tube fuselage..
the same techniques would apply to a sports racing car chassis... with some adaptation to the TIG welding process.... as opposed to oxy-fuel...

http://www.youtube.com/watch?feature...&v=Q6q1VKsTeKQ

It is instructive to see how effectively things were done some 70 years ago... before hi-tech

This link had been posted on SFT... credit where due and kudos to diggerdoug..