You will need to get a bronze spigot shaft bearing turned up by an engineering company, this goes into the end of the fire crank, I have a sketch somewhere but since I moved office I can't find anything! Still its an easy thing to work out, it needs to be a snug fit into the end of the crank but also a snug fit in the end of the gearbox shaft (note: the id will reduce slightly as you knock it into the end of the crank)
You also need to think about the flywheel, we use Panda 999cc flywheels because the clutch is slightly smaller than the later flywheels so the bell-housing of the gearbox does not have to have too much material removed from it.
I also have a pattern for casting an adapter plate and a set of transfer punches and pattern for transfering the mounting holes. Its not the easiest adapter plate to make as it needs to be around 30mm thick (i can measure mine if you like) thick so the choice is either to use a big thick alloy billet, cast in alloy or cut in 10mm steel plate but add on 20+mm spacers which will work but leave a big open gap between the gearbox and engine.
It is possible to do the conversion in a few days if you have all the bits prepared and ready to fit. I spent quite a long time planning and fabricating all the required elements and started be getting the engine and gearbox mated together on the bench before I even thought about how to fit it into the car.
There are plenty of photos on my
www.retro500.com site, have a look in the gallery at 'nigel1108' in the build pics section.