Hi Ray:
First of all, I'm not a blower expert, but assuming that you have a good sealing 6-71 & are running it at 1:1, you should be able to make about 10 lbs. of boost at 6000 rpm's. A 10:1 static compression motor with 10 lbs of boost requires some HIGH octane fuel to combat detonation, not the crappy 93 stuff & "O" ringing the block may not be a bad idea. Why is the boost falling off? Well, the single 850 DP carb maybe restricting the amount of fuel (or air) you require & most importantly, you could be leaning out the motor (you usually do this just one time). HEAT is your biggest enemy & you are building lots of heat in that motor. What do your plugs look like? Any backfiring?
Maybe Pat McCarthy can throw out some other ideas.... Don