Maybe it's just my engineering side coming out, but after building a LOT of different projects, and working in that kind of industry,
AWESOME AWESOME MAN I LOVE THAT KIND OF STUFF! 
planning for every failure gives you every chance of success.
BE PESIMISTIK
It's good to see you already have plans protecting the frame, though for steel I would avoid powder coating,
HAVE POWDER COATED STEEL BEFORE WITH NO PROBLEM as this can trap moisture and promote corrosion. Galvanised + Paint is better and cheaper in this circumstance.
THANKS THAT IS PROB WHAT I WILL DO GALVAIZED AND THEN HAND PAINT FRAME AS NEET AS I CAN ALSO I MEANS I CAN DO SEVERAL COLOURS
So you don't think I am all negativity
NAH I SEE YOU POSITIVE AND ENGOURAGING SIDE NOW
http://home.tallships.ca/mspencer/winmiter.html
You may find that useful, it creates paper templates for you to cut around for fish-mouthing tubes to but up against each other
THANKS VERY USEFUL BUT I THINK THIS WILL PROB BE ALL SQUARE TUBE FOR SIMPLICITY
I wish you the best of luck with it, and look forward to seeing progress, just heed my advice, question every step of the way "what can go wrong? Am I using the best method I can?" And it will be a success, make no assumptions, and go at every stage with cold hard logic.
THANKS