sumped tank issues and questions..

[Josh]

I sumped the tank in my 79 Caprice and have the outlet near the front of the tank, passenger side, lowest part of the tank when on level ground. I am using a Barry Grant pump on the framerail just ahead of the tank. The tank is stock and has no baffles. I have, at times, heard the pump try to lose it prime when turning right, mostly when the tank is down to the last 25%. I keep it more full and this is not too much of an issue. My issue is this: I get the timing set just right using dial back light and highway seat of the pants testing. A couple of trips into town later I decide to let it rip and the engine chatters like mad. Then I feel I have to re-tune everythinig and so and so on. I am wondering if I am going lean due to slosh of the fuel in the tank to the back when I go to WOT, thereby causing detonation that would not nomally be there at high RPM without undue acceleration? This is a BBC 540 so it accelerates well. Either that of the BG pump that is rated to 1200HP is wearing out?
Any input appreciated!