Shaun,
You asked, if useing round tubing what wall thickness should I use...Well there are many variables to consider...but I will just try to give you a general idea...
First , as Russ said you need to decide if you want the ultimate light weight chassis for racing, or a reasonably light chassis for the road, that will last for some time.....
, The quality, and diameter of the tubing you are useing, will dictate the wall thickness, if you use chromolly 1" by 16swg (1.6mm) 4130, then to get the same strenght from 1" cds2...cfs3...dom...etc.....you would have to go up to 14swg (2mm)....(This is just an aproximate comparison) If you were to use 14swg wall, your chassis would be pretty heavy..
The diameter you use will depend on your chassis design. If I was making a space frame from tubing I would use chromolly one inch, and an eighth (28.5mm) x 16swg (1.6mm) for the majority of it, and other diameters where needed
chromolly comes in 4 different conditions
As Rolled
Anealled
Normalised
Hardened and tempered
I would use Normalised. If you Tig weld, or Braze it you wont have to heat treat it afterwards, but if you Mig weld it, you will need to nomalise it again by heating it up, and allowing it to cool at room temp, If you were to pre heat it before Mig welding, then you probably wouldnt need to normalise it after welding. There are a few web sites where I found this information, if your intrested ILL look them up for you and send you their addresses
The best way to help you decide, is to look at other peoples chassis, and ask them what they used. If you look at the forum about Can Ams, and Porches. Just above the paddock forum, you will find some space frame chassis there....... Here are 2 that I just found
David Sturch is building a Chevron B16 Replica
Chris Melia is buiding a Porsche 908/910
I hope this help you
mick