You can attach the ground wire right in the trunk. Make sure it connects to a captive nut welded to the body, or a good sized area cleaned of all paint, undercoating, and primer. Either way, thoroughly wire brush the ground point so it's nice and clean.

Then the engine needs to be properly grounded so it can start. You can use the existing ground cable, cutting off the battery end, add a large terminal lug, and bolt it to the frame. Again, clean the grounding point to bare metal.

I would also protect any new connections with grease to prevent corrosion and loss of conductivity.

For the positive cable, I think 4 AWG wire will be sufficient. Again, clean solid connections are the key. You need a properly sized, properly crimped terminal at the battery. You don't want any splices in between battery and the starter. It'll be an 8 to 10 foot cable so get good stuff. Bigger is always acceptable with this kind of thing.

Run a separate 10 AWG wire to the front for all the accessory and distribution wires. I'd add an inline fuse holder (30A, 40A, something like that) in the trunk for the smaller cable. Both must be properly secured and sheathed to prevent abrasion short circuits, and FIRE. Have a look at the systems built in race cars- they're a great example of successful installations (usually!).

Good luck with it!
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Rob Bareiss, New London CT ::: 87 244DL, 88 245DL, 90 745GL, 84 242DL project, 89 244 parts, 88 244DL to replace the 87