I've replaced both cables on my '86; if yours is like mine, you have a rather long run for the positive cable from the battery to the starter - on the order of 72-76 inches (2 meters) or so, IIRC. With that length of run, a 2 gauge cable is better than 4 gauge (although 4 gauge will work, with the expense of a slight voltage drop to the starter).

You may want to look at a farm/ranch or truck/tractor supply store - I know I've seen long 2 gauge cables at the Tractor Supply Co. chain stores. As you've found, the auto parts chains don't usually carry the heavier gauge cables in long lengths. The Volvo dealer cable will (obviously) match, but will also be very pricey.

I think brand name isn't important so much as build quality - good insulation, well-crimped terminals, and a good battery terminal clamp are desirable. Length doesn't have to be exact, although a few inches longer is much preferable to a few inches shorter (if in doubt, go for the longer one...).

As I found when buying mine, the parts books aren't always accurate - they'll probably list their closest size to the OEM, not necessarily the exact size, and if they round down, they may list one that's actually an inch or two too short (happened to me).

Regarding the terminals, an exact match isn't always necessary, so long as it's close; the ID of the ring terminal at the starter end of the cable should be a good match, and the battery end should have a 10 gauge or so "pigtail" attached in addition to the main cable. If it doesn't, it's easy enough to fabricate one to attach to the clamp bolt, though this not quite ideal, since it provide another possibility for corrosion in that connection over the long term. If you have a top post battery, be aware that the terminals may have sightly different diameters; try to get a cable with the proper size clamp (the wrong one can be made to fit, but again this is not ideal) - I believe this is the main difference between cables labelled specifically for positive or negative use. If the cable isn't specifically labelled, it probably has a "universal"-sized clamp which will fit either terminal; sized midway between the two diameters, and is opened up for one and tighened down for the other.

When replacing the cable, I placed some additional covering on the cable where it passed across the front of the engine, along the front crossmember & beneath the crank pully as a preventative measure against abrasion and/or oil leakage. I used some of the corrugated plastic "wire loom" split tubing, although heater hose, vinyl tubing, etc. slit lengthwise would also work.

Overall, it ain't rocket science - get a good one that's a close match and you'll be okay.
--
Tom Harper ('86 740GLE, NØMKC)