Could you use a small size of Tube Metal ? Answered
You could tack/spot weld a small size of tube steal a (1/2" or 1") underneath the bottom side of the angle iron @ the 90 degree bend. This would make the Lathe rails/bed a lot stronger. Tube Steel would be the best option because it wouldn't bend or Flex like Roll Steel might.
You can hold a long say a 10' piece of roll steal up and you can see it bow some. Then hold a 10' piece of Tube Steel up and compare the difference of bow in the two. Angle iron would be a second choice.