This is going to be a back burner project as this has never been done before on an XS400. I've decided to try another approach to fitting the charger and this seems to fit (just)

I may get away with a 90mm pulley system which will make the crank fitment a little easier. It will aslo facillitate the jockey wheel and leave enough space for the belt to be pinched. the fuel will enter the top aprt and exit from the bottom Mount/ manifold. not sure whether to have 2 out lets either side (which looks good)or go for a single side to plennum chamber