"Art Newland" - Even I have gone on record saying that you shouldn't even consider using anything but straight linkage on a manifold like that. I have to believe that the only reason Charlie Price recommends progressive linkage on one of those is because he makes more money on them than he does on a simple straight linkage setup.
"Mart" - Thanks for your contribution. It's valuable to hear from someone who has actually tried it. (I'd like to do some experimenting myself, but I've never really liked those things and would't spend the money.)