Earthing direct to the battery is the best way as it will eliminate any resistance which would be caused by earthing to the body!

By earthing to the bodywork you are going to have a higher resistance than going direct to the battery.

Ideally you want to be seeing 13V+ with no load, but around 12.8-13V under load is ok, the fuel pump is a 12V device, so long as it has this power UNDER load, as this is where the power demand is, then your fine!

Martin