All the comments on "insurance", problems avoiding warping and just buying a factory issue long bar would seem wise to me, but something tells me you'll give it a shot anyway
So, if you decide to live an adventurous life and go go for it, possibly you could consider cutting the bars lengthwise and opening them up a little toward the center of the "new" bar to create a belly. This will also allow you to use more of the "almost parallel" (but not quite) length of the original bars. You'll need to do a bit of a fancy "T" or "Y" cut toward the tip and base to avoid warping the metal too much there though while bending the bar open, or work with 6 separated parts (base, tip and 4 length of side), but it will give you more flexibility in assembling an optimal line of guidance for the groove. No need to completely fill the "gap" you'll create in the center, just some bridges to keep everything together - plenty of hollowed/drilled light weight bars around.
Also, use material thinner than the bar for the bridges, so you have some "L"-shaped joints to fill on each side of the bar/bridge, without the weld sticking out of the original bar surface. If the bar is say 4-5mm, use 2mm steel sheet for the bridges or some such.
Like ATpro mentioned, the real challenge will be to keep the whole "project" aligned in (mostly) flat plane. Spot welding a limited amount of bridges (as opposed to full length welds), putting in less heat and allowing to cool between welds may also help with that.