Don't overlook the old "get rid of the mechanical pump" alternative. If all else fails, an electric pump mounted near the tank, with a filter inline, and a blockoff for the old mounting on the block, will fix the problem. Also, have you checked the rest of the fuel system for problems? Are all of the rubber hoses open and not collapsing somewhere - sometimes the lining inside the hose will separate from the other layers, and collapse and inhibit flow under suction; is the pickup in the tank open and not plugged up; tank clean; no clogged hard lines; filter in the carb clear?? Sometimes it's the little details that can cause you the most agravation.