hi rec,
before i did the operation i was in contact with a fellow in england that made 130 and 135 mm conversions professionally. he had widened a number of brompton steel rear triangles without problems, had made a jig for it. when he tried it on a titanium version it unfortunately cracked, in its welding.

so i would cation against a frame spreader. maybe if you have two extra of them and mount the two extra to protect the forward part of the tubing while the third makes the spreading. be careful.

so i instead bought a set of pipe benders, they were also slightly expensive the tactic was to make the bend along the pipes, not their roots. worked beatifully, the tubing was softer than expected, and there was no problem to bend the dropouts back.

made like 5 small bends right next to each other, just so that the metal would give, alternating between seatstays and chainstays. afterwards it looks like just one bend.

when doing this you have to bear in mind that the lower part of the triangle is asymmetric. the right chainstay is more medial than the one on the left, and there is a slight indent on the right stay to make room for the tyre. if i remember correctly i had to increase this indent 1-2 cm forward, because the wheel will sit a little more forward after the operation. this also means there might not be enough room for schwalbe marathons; standard bromptons work fine, but there is a slight squeeze against the bb tube when folded.