Slant header ID?

Now see? IF You'd only angle-milled the head for maximum squeeze, You'd be golden! Lol!
Odd there isn't one stamping anywhere, but it wouldn't take much of a tweak up top to make 'em move a bunch below, I'd wedge a block somewhere & heat the tubes near the flange 'til it relaxed. May take 2 or 3 steps, IF they actually fit the chassis they're destined for. Paint be damned!