I mocked up my rear suspension yesterday. I have welded bottom brackets, fully built wishbones, but the top brackets are yet to be welded on. Clamping everything in place according to the measurements and I'm about 3mm out of position (front to back in relation to the car). So as I lower the front wishbone in place, the top hats are in line with the bracket edge. If I push them in place, they're slightly out of line with the brackets.

I suspect what's happened is that I've mismeasured the MX5 uprights, and the top wishbone is slightly out of position. However, since the top brackets are the only things not built up, I was wondering what would happen if I simply moved them back a couple of mm to line up?

I can absolutely see that if I did the same on the front I'd be effecting the caster angle, does caster apply to the rear suspension? Even if it does, since I'm not moving the rear upright (only the inbound suspension end) is it changed? Is there anything I need to consider if I do move the brackets back a bit?

I'll post a picture later which should make it a bit clearer.

For 3mm, it'll be fine to just move the brackets backwards a bit. Caster only really has any effect on steering, so unless you've built in some rear steer you'd never know the difference.