Sailun,
"My question concerns the existing body ground mounting stud on the firewall. Is this a comman ground collection point, like a bus bar, for all the stuff inside the cab, or is this just a stud on the firewall ?"
On my '95, a ground stud is located on the passenger side firewall and one ground strap goes to the head & one to the chassis. It is positioned just underneath the heater hoses that run into the cab.
When removing the nut to replace the well-worn OEM straps, the stud snapped off halfway down the thread. I pulled the floor mat back and discovered that it is not a common ground to anything other than the engine-cab-chassis.
I drilled it out and replaced it with a stud/doublenut/star washer combo and also replaced the straps with 4ga wire. OHM meter tells me that there is zero resistance at numerous points, so I think things will be OK. Everything will then be painted with Rust Bullet to prevent corrosion. I'd post pics if I knew how. Hope this helps.
If there are other ground studs that you are wondering about, let me know & I'll look around as my rig is still in pieces.
-Bill
95 K2500 6.5TD, Ext cab, short bed, 123K, FSD Remote, minor overhaul @ 123K: new injectors, glow plugs, glow plug relay, injection lines, heads, exhaust manifolds, harmonic damper & idler damper, water pump, (timing chain at 3/8" play) 4" exhaust, K&N air filter, CDR valve, trans cooler lines, alternator. Dead Kitty & deactivated EGR. (MPG: 11.5 before work listed, 13.5 afterwards & 17 w/ Marvel Mystery Oil)