Originally Posted by Road Fan

There were 6's which have the same distance between sprockets as Regina 5's. Those 6's require 126 mm hubs unless you go with a radical and possibly risky dishing.

There were SunTour "Ultra-6's" for which 6 sprockets were spaced close enough together to fit into the same space as a standard 5-speed. Those can use a 120 mm hub.

From SunTour and others there were 7's with spacing to fit into a 126 mm hub, and some others which were a little too wide and needed 128 mm hub spacing.

So to the OP: measure the spacing between your rear dropouts (inner face to inner face). Most likely it's 126 mm, but it's worth checking. It's nearly 40 years old and could have had a lot of bumps and bruises in that time. It's also worth doing a string test to get an indication of whether the rear triangles are evenly offset.