Re: replacement of glazing. (historical)
This technique only works decently with 1 3/4" sash, otherwise you are milling too deep into the glass rabbet.
My local glass shop will make insulated panels. Cost for 6-over-6 (or more) panes is going to quickly add up.
Then, you're weakening the small muntins, maybe fatally. You're losing the wavy glass. And multiple panes is multiple chances for seal failure = foggy lites.
The money, IMO is better put into high-quality metal weatherstripping, storm windows, and insulating curtains.
Remove not the ancient landmark, which your fathers have set.