Thats basically exactly how I did it right there Gareth, patent pending "Peter's Chassis Repairs Encorporated". The holes were drilled approx 1 inch apart, on both sides of the rail for the entire length. You can see the drilled holes on the previous page. I went through and spot welded along the edges of the rail afterwards. The RHS at the end might be a bit bogan for some, but it was the best I could do without the car on a hoist and will still be very strong. I fish oiled the rails, then sealed the seams and welds with sicaflex, then painted body deadener tar over the sicaflex and painted the rail itself black.
So much easier for those of us not blessed with a hoist or with their car on a rotissery. Although I still had to weld each end of the rail under the car still, which I still have a few bald spots on my head from. At least the smell of burnt hair has cleared the garage now.