are you welding with tig or mig?
If you doing a tig weld I'd do what AydinZ71 suggested and run a single pass or as close to one as possible. I have no doubts that it'll warp but just take a dolly and a body hammer to the tig welds and hammer them into themselves. You'll restretch the metal and it'll flatten out again, then you can grind down whats left of the welds.