Chris it sounds like you're on the right track for most of your comments here. I've run a 671 on a 355 small block for 20 years on the street with occasional strip use. Your compression estimate is about where you want it. Same for the drive ratio. Depending on the chamber cc's of the heads, you should be pushing @ 10 psi of boost at 12% under drive. That's about where you want to be on a street engine. Roots type blowers have a hard time making more boost since they start to create heat with added boost which begins to diminish power. Hopefully the previous owner installed thick top blower pistons like the ones TRW offers. They are also coated. Another good tip when building a blower motor is to go with a plasma-moly top piston ring for the added heat you'll deal with. This ring requires a special cylinder hone.
If you have aftermarket heads on it you are in a good place as this is a big area for making more HP on a blown engine. I've been told you want to avoid a polished port though as the fuel tends to puddle up as opposed to remaining more atomized with a slightly rougher port finish.
The cam is another big area to build HP on a blown engine. As far as cam grind; talk to Dyers, BDS or Weiand for suggestions as it depends on a couple of parameters. A blower cam should have long duration with lift being less of a concern. You have to watch your overlap though.
There are some good books out there about street supercharging and they will include chapters on how to build/tune the carbs if they're not already done. Again that is another area that a blower shop can help you with. Basically speaking, the carb areas that need attention are jetting, power valves, some air bleeds and if you run vacuum secondaries it's best to run a balance tube between to the diaphrams. Also as mentioned you have to set up the curve on your distributor very carefully to avoid detenation. Roots blowers are always creating boost and have no lag time like a turbo when you mash the pedal. You also want to avoid vacumm advance on a blown motor. MSD is manditory for a nice fat spark. The more advanced units can set up timing retard based on RPM's and boost.