I've looked into this before. There's two fairly easy ways to do this, the first is to cut the B pillar out of an early shell and nab the doors while you're there, and simply weld that into your later shell.

How do you stand legally doing that? Would there be any problems at MOT time?

The slightly more fiddly way would be to get two B pillars from a later car and cut the rear door hinge off and weld it to your B pillar, then weld the door part of the hinge from two old rear doors to the trailing edge of the front doors, arse about fitting your catches and Roberts your mothers brother.

A nice idea we saw in Czech was to have a hook and latch that allowed you to hold the door open an inch or two whilst driving, getting the air flowing in.