Thanks to everyone for their thoughts. I may end up having to use loctite to retain the bearings, it all depends on how accurately I make the housings, but that depends on what diameter I should aim for, as stage one.
Having read oodles of technical data from various (bearing) manufacturers, the amount of info is considerable, so I was rather hoping someone may have had a similar job to give an initial diameter to aim for. At present, choices are limited. If slightly too small and welding distorts things a little, it will never rebuild, so trying for a press-fit is beyond reach.
They say a thread is worthless without pics so, here's a some pictures to show the starting point...