It seem's like you can do it this way with 3 different options.
(1.) Do this but don't chassis ground the pump leave the OEM ground going threw the OEM harness.
(2.) You could run a whole new ground wire directly off the controller harness to the pump. To keep the current up as high as possible while still maintaining the fuel pump controller. That would eliminate the current drop (if any) caused by the harness on top the sending unit.
(3.) Instead of chassis ground on relay. It could be ran to FPC ground that originally went to the pump. This way the pump can have equal gauge wire on + and - side helping with current.
Witch one of these options would be best? I'm thinking Option (3.) though I'd prefer option (1.) being how lazy I can get!
I'm also thinking option (3.) would burn up the relay I'm not 100% but would the relay need that constant ground to operate correctly?