If your downtube shifters are Indexed Superbe PRO, the right will have settings for Ultra and regular spacing. If you can find an ultra 7 freewheel you can simply replace the freewheel and chain. If you can only find a used one, I believe you could re-use 3 or 4 of the COGs in your current 6-speed with the spacers from the Ultra-7.

If your Suntour parts are Superbe PRO, think hard before you abandon them. Back in the day when Suntour became extinct, I was averaging about 0.75 mph faster on my Suntour bike than my Dura Ace Bike, all else equal. Although the Dura Ace bike shifted better under extreme conditions, the hyperglide stuff used angled teeth in the cogs and bulged outer plates on the chain. Although this would change cogs eaiser, it was noisier and created more drag than the Suntour bike.

I ride my Superbe PRO / Ultra 7 bike to this day with straight cogs and a Sedis chain with bulged inner plates. It is very quiet and smooth even if it does require precise adjustment.