The correct way to fix this is to heat the ring gear with a torch and reinstall it , make sure it is in good shape though, no sense in reinstalling a bad ring gear, afterwards a few tacks with a mig or arc welder at opposite sides should hold it in place untill you need to grind off the welds to replace it in the future. most of those old ring gears were normally heated cherry red when assembled and as it cooled it contracted hence "press fit" , It would be advisable to actually remove the flywheel when doing this that way you can make sure the gear it is seated correctly and fully on the flywheel,