Are you looking at a new Stingray or an old (pre-1985) model? This makes a difference but I'll try to answer anyway.
Older Stingrays used Sturmey Archer AW 3 speed hubs which were 120mm wide- the same width as an older 5 speed hub, so interchange should pose no problem as long as you remember that you'll need to change chain, cables, shifter, and add a derailleur. You'll probably want a different chainring up front too.
Newer Stingrays- to my knowledge- use Sunrayce hubs. Sunrayce now owns Sturmey Archer, but it is my understanding that they have re-engineered the hub most often used on smaller bikes such as the Stingray... and it isn't the AW. IF this is the case then the hub is likely 135mm which is the current width of an 8/9 speed rear and it would not be a good idea to "squeeze" the stays to accomodate the 5 speed rear.
All this being said, I haven't actually looked to see if Pacific utilized a Sunrayce hub on their Stingray 3 speeds and if they haven't it is a total toss-up and you'll need to measure on your own.
Bookmarks