I always use standard tie-bar bushes and change them often.
The tie-bar 'ears' must be strengthened with a triangular piece of 4 mm thick steel welded to the 'ear' and the sub-frame web. It's obvious how to do it when you look at it.
On a rally car the suspension needs to be a bit compliant and not 'tight' as it would be on a track car. Too many people set rally suspension too hard and find the car won't hold the road or handle as they like. The suspension needs to 'work' on rallies and the Mini especially so. The available suspension travel is small to start with, so raising the ride height slightly and fitting compliant suspension parts does help. When one of the staff reporters from Mini Magazine drove my 'S' several years ago he remarked at how relatively soft the suspension seemed compared to some Minis he had driven. I use standard OE spec rubber cone springs rather than the competition ones as the initial rate is a bit lower and I find this works well.