My engine needs about 16 psi of boost to make 800 hp. You may need a little more boost with 6X heads but it's a good starting point.
With 16 psi of boost + 7 psi for the carb you'll need 23 psi. (as tininjun already mentioned)
The BSFC for a turbo engine is around .60 lb/hp-hr. This works out to 480 pounds of fuel per hour for 800 hp.
Converting to gallons yields about 85 gallons per hour. (at 23 psi)
The maximum pressure output of a Holley Blue pump is 14 psi. Therefore it won't cut it.
You can follow this link to the smallest Weldon pump I found that will support the HP you're looking for.
http://www.weldonracing.com/a2005.html
If you look on the "performance chart" it shows about 87 gallons per hour at 23 psi. I'd recommend buying a bigger pump, that way you can crank the boost up to 22 psi and make 1000hp.
I can't help with carburetor questions, Tom V and PontiacDude probably can.
PontiacDude - Do you mean the MagnaFuel ProStar 500 pump? The information I found says that it free flows 95 gallons per hour. I haven't found any information on its flow rate at 23 psi. Have you used it with 15 psi of boost? If so then it's a pretty good deal.