Interesting . I would think 3/8 bar would be pretty flexy. I think 1/2 inch can still get in most spots.
The pro spot guy recommended heating to 450 before pulling, he said at that temp , you can see it expand and start to rise.
I have some 1/2 round 1 inch slugs that we used to cap ends of frame, and some 3/4 inch I thought them tacked to the side of the bar, would be good to push against.