spacingwise you shouldn't have any issues going from an old 6 spd to an 'ultra', in fact you should be able to fit a 7 spd ultra - this assumes that your rear dropouts are 126mm spread. If the rear is actually a 120 mm, then the current 6 spd is already an ultra and is the max you'll likely be able to fit without coldsetting (steel only) to the wider rear width.
Other consideration is of course, the cog size range and what your derailleurs can handle.
and rarely, peculiarities of dropout config and derailleur could cause an individual anomoly, however that would be rare.
I can only testify as to how an older friction system works with freewheels in the 12-21 range and a few that I have used up to 24. I noticed no ill shifting in going to 7 spd ultra spacing for all my 126mm bikes. Freewheels with larger cogs - prolly OK, but ask someone who has that experience.
And having upgraded to 'modern' technology by using Suntour accushift on my daily rider (an old index system), this also works quick nicely on a 126mm, ultra Freewheel based system.
SRAM pc-58 is 'Da Kine' - works v-nice for me, now that I've run through my entire stockpile of Sedi Sport chains.