Which frame builder will cold set the rear triangle for a tenner. Argos for example are £30.
back to the original OP, great idea doubt you'll get many nahs on this forum about using a steel frame. Nowt wrong with race blades or similiar. Never could get on with guards in the old days so went without, justt got some over-trousers
Watch out for most 531 being 126mm spaced, I would say that as you are after a race orientated frame, that by the time that 130 became the norm, 653 became more attrative than 531 and alloy or early carbons started to filter through.
You can spring a 126mm to 130, after all its only 2mm on each side. just makes re-fitting a wheel after a puncture a bit harder than it needs to be.
do not despise the snake for having no horns, for who is to say it will not become a dragon?