I selected the Talbot lotus to try and sort out because it had the least clearance and the tyres are close to the gaurds so fitting larger diameter tyres would be hard.
The clearance is less than 1.4mm under the centre of the chassis.

The brass wire is hard to see in my crappy pic!
The chassis was a moulding under the axle that COULD be sanded away.

I removed the rear axle and simply filed out the bearing hole 0.5mm lower.

This lowered the axle in the chassis giving plenty of clearance. A little too much!
The brass wire in this pic is 1.6mm

Once I was happy with the clearance I simply added a drop of super glue to hold the bearings in place.

I had that much clearance that I could sand the tread off the original tyres to make them slick.
I then lubed up the crown gear and ran it for about 10 minutes to bed it in now that it is running as a hypoid and the car is fine.
The crown gear on the axle STILL has only 1.2mm clearance but the rest of the chassis clears the rally track very well.
I also looked at a Fly Rally Porsche and IMO the best solution with those is to simply sand or file away the bottom of the magnet pocket,as the rest of the chassis has plenty of clearance.
Not too sure how these will go with so much weight behind the axle!
















