one of the things I ran into was fuel feeding into the carb...
my old holley 600 was a single feed and just wasn't up to the task (I know your 850 is dual) but if you are using one of those one-splits-into-2 lines, that _might_ be a source of trouble.
I'm plannning on some major upgrades down the road, so when I put my new dual feed carb on, I used a holley pressure regulator mounted right next to the carb, the regulator has massive ports on it.. I want to say they are 1/2" NPT, but I might be wrong, I know they are HUGE
This is the only pic I have of my regulator setup. look closely at the passenger side of the carb.
so right now I just run the 3/8 line from the pump to the bottom fo the regulator, then then each port on the regulator feeds the two float bowls, for me right now, it's over kill, but eventually I will be feeding the reg with an-8 from the pump and then two -6's to the carb :>
it's possible that your fuel lines AFTER the pump may not be up to the job.. just a thought.. gotta think about the whole system.