Hi
it can be done by creating the radius on a three roll bending machine but since I assume that you need only 2 pcs. you can make it manuall - the "primitive" way, by cutting slots in the tube and then bending the tube to the desired shape ,welding it and grind it clean.
Clamp the long bit (to thr right of the bend) to an immovable object with everything you've got, then before you cut the other end to length you can bend it manually with a long enough 'arm'. I'd bend it round a former, or just something at the point of the bend. You'll only bend it a few degree per 'yank' so just keep an eye on the angle as you go.
Cut and mitre it or alternately heat and bend it ().If you only have this chassis to do. I've got a bender, but to buy the dies in that size would cost £350.00.
Took the guard off to get into the tight corners, disc is correct for the grinder, I do generally run with one on but sometimes I find they get in the way. Suitable protection was being worn lads, I've had a nice metal splinter pulled out of my eye in the past and have no wish to repeat the experience!
Bigbadbugga, yes, rivnuts, although it's been suggested that it would be worth my while trying to tack the outside to the steel as they can tend to separate internally.
I would agree with the tacking of the rim of the rivnut.
I'll be making some channels for my Scooby powered bug in a similar way soon, but using 50x50mm for the front section and 50x100mm for the rear section. I was thinking of using a GSF supplied closing plate though by just drilling the box section for the inserts in the closing plate and welding the closing plate to the box section.