How is your high pressure line connected at the pump? When I did my saginaw swap, I had the same issue. I was to lazy to make a new high pressure line, so kept the old one with the o-ring flare and swapped the fitting from the ford pump to the saginaw. Went threw 2 pumps before I realized that I needed to make a new line without the o-ring and use the gm fitting at the pump.