Worth the effort?

Quote/ I bought the gears new as 3.55's and cruiseing on the highway at 60mph rpms are around 2800.

Yea that's about right, my 3.91's are running 3000 at 60mph with 255x60-15's. The dia. on the 255 is 27.1" also.

I'm starting to lean towards cam timing also, that cam is advertised at exhaust fully closed at TDC. You could pull the drivers tappet cover and eye no.1 exhaust to just closed then look at your TDC mark on dampener to chain case to see if they line up. Maybe she's out by 10 deg. or some ungodly number. Maybe who built that engine advanced her (or I could be bass ackwards) to far to give you a little more vacuum at idle, who knows.
Failing that I'd see if you can wangle another 650 DP'r out of a friend and try that. I've got an old 750 vacuum secondary on the shelf but that ain't going to help ya.