How to get a good ground in your Mopar
which was welded,............ which was welded ............40 years of rust and corrosion .................... Welds can fail and in unibody structure.
If all this welding, rust and corrosion is leaving parts of the unibody loose, then grounding is the last of your worries. The front end will evidently fall off the car.
you are likely correct that you may have difficulty measuring a drop across the unibody proper, but the difficulty is in providing an adequate bond from the cables to the sheetmetal.
The above I believe is the heart of the argument. 00 or larger copper is not cheap. Bonding the main battery ground to the trunk is just not that difficult. You can buy lugs with two bolt holes, or even weld/ silver braze a bolt/ ground plate to the trunk floor.
I don't have the money to go out and buy a whole bunch of cable and re-do testing that I did in 1972, but ohms law hasn't changed since then. Neither has the integrity of the unibody -- if it's still safe to drive.
The point is, just how far do you need to carry this? We are not talking about the space shuttle, here.
Tell ya what. In about a six months I'll probably be ready to trunk mount the battery in the 67. I still have a couple of great big carbon piles, and between the two of 'em I can put about 1000 amps of load on the system. Remind me along about September, and I'll do some serious testing.