1997 Suburban k1500 5.7 L:

I have been struggling to fit both English and Metric hex sockets over the grounding screw that grounds the fuel pump to the cross frame beam .
Seems like 15mm is close , 5/8 is close.
What works for you folks?

What size hex for fuel pump grounding screw?

 

Looks like it is pretty rusty and may have had some rust away making it a bit smaller. I would use vise grips or a drill as a last resort and replace the bolt.

 

Sure looks to me that the wire connector is in the way of getting a socket onto that hex head. I'd pry that connector away from that grounding bolt to get clearance.
Since you say that it's a grounding wire for the electric fuel pump, couldn't you just cut that wire and ground it somewhere else? What's under that electrical tape - perhaps someone before you already simply cut that wire and then did a "repair" under the tape.

Seems to me that my 1994 Suburban was "mostly" metric, but there were still a few "standard" size nuts and bolts remaining.