After having got the 7.5" diff/beam into the car, my next problem is how to assemble the hubs. I thought that would be the easy part.....

So being a bit cheap, I thought I'd stay with drum brakes at the back as they are fine. I've managed to fit the drum back plates to the hubs by cutting off the caliper mounting lugs and by enlarging the centre hole slightly. So no that is not the problem!

The problem is that the drive shafts will not quite go into the trailing arms. They very nearly do, but not quite so far. It seems that the bolt flange is scraping on the top side of the arm. This surprises me since although clearly it's a non standard setup the standard one for disc brakes was I thought 108mm whereas my ones are 100mm.

Are the Trailing arms on a Cosworth etc any different to the drum brake models?

It's so close I wonder if a large hammer would fix the problem
Here is a drum brake back plate fitted with an ex-disc brake hub. It was all looking so promising until I tried to fit the 100mm drive shaft!