You can buy true mandrel bent aluminum tubes from Burns and probably others. When I did the Elva's plumbing sometime ago my boss got them from a place in Long Beach (I think) called L&M Bending, or something close to that.
If you have an alloy choice, go with 5000 series or 6000 series, not 3000 series.
"It only seems kinky the first time"
-- Bumpersticker seen in Lost Wages
If you put your radiator in the rear make sure you properly connect to hoses in the right place. Some people have crossed the lines and ended up blowing a couple of motors because of it. Just be careful