Just yesterday I replaced the Campag Rally (1st gen) on my 5-speed (14~28) Peugeot with a Duopar Eco and, apart from it being a complete REVELATION about how fantastically well an RD can ACTUALLY shift, and the shame it puts the Rally to, I found I had to screw the inner limiting screw in almost all the way, with around 7mm of thread showing, while the outer limiting screw still has about 1mm of thread showing. Based on this, and the fact that the cogs on an 8-speed are closer together than on a 5-speed, I would be very surprised if the Duopar didn't have the range you are hoping for.
You probably know the Duopar but riding the Peugeot yesterday I was completely in awe and thrilled. What a fantastic derailleur! I've been thinking of starting my own thread about my experience as I'm convinced that anyone looking for a vintage friction RD for touring should get one.