In regards to the BAC mono that is a true spaceframe with a stressed engine and the body work is just that.
BAC used round tubes because per weight for torsional and compression strength, which is generally what you will be aiming for lower weight with higher strength.
You will find most home builders (GT40s, Locost's) will use square tubing as its much easier to fabricate and thus makes the build quicker.
If its a 1 off i would go this path and take the minimal weight hit as fish mouthing is a PITA even with the tools vs cutting flat edge on a SHS unless you get all of the tubes CNC cut.
What you can do to save weight is delete the need for stiffening between frame sections by adding panels to make in in shear. This can be done in a number of ways ie AL sheet glued and riveted on, using AL honeycomb as filler, welding in a steel sheet, etc. This is taking steps towards being a monocoque without trying to workout fatigue in AL which is not a space to without real world experience.
Check out:
http://www.themotorreport.com.au/166...s-the-inverter as an example of how this can be done - all plans are downloadable. Not a cheap car to build as too many custom parts for my liking but then again i operate in the extremely low end of the car building $ scale.
I did a uni project which involved an electric F-SAE car, a number of years back now but range/performance/$ did not come easily even for our modest targets.
Cheers,
Taffy