If I was to use your method of installing the lower head race I would do that first using the old bearings and races everywhere else.

Eg remove lower head race
Cut slot in lower head race.
Position new head race
Place old over new and clamp into place.

That way only old parts are under pressure and at risk of damage. The only contact between old and new race is on the non bearing surface. And you can remove the slotted race very easily with a slotted punch.