Only one battery as I recall, so that simplifies things a bit.
I been using Carol brand welding cable exclusively in battery cables. Cable lugs crimped (only) with a Greenlee hex die crimper (that a Licensed Electrician friend gave me on long term loan). Covered with Ancor adhesive lined heat-shrink. Been making them this way for the last 15+ years without any problems.
Welding cable is a far better conductor than a welded steel frame. I've a deep dive observation about why, but I'll leave it out for now. IME over the last too many decades to think about those vehicles with remote battery's that had persistent hard starting problems all were grounded near the battery. Those that I've built since making that observation have all been grounded at or near the OEM grounding location (usually the engine block) and haven't had that problem. Old school starter motors vs. PMGR starters are also a factor in this.
If you want to put some isolated posts near where the battery used to live so as to disturb the OEM wiring as little as possible I would use one of these Blue Sea Systems
Dual Isolated Battery Post. I used one of these to tap into a 6 ga. pair of camper charging cables for the OBA compressor on our CTD/Camper. That brings up a topic worth mentioning. Charging the battery from the alternator and the attendant voltage drop from the long(er) total circuit length. If you want less than 3% voltage drop (I do) then the charging cables can get quite big, way bigger than just the current would require. Since you're sizing for winch power I doubt that this aspect will be the dominate cable sizing factor, but if you didn't have the winch it easily could be. There's downsides to going ridiculously big, but a bonus of slightly oversize cables is that the starter motor sees less voltage drop, so it also performs better.
I use manual marine battery switches when switching high current cables. They're not as convenient as a contactor, but I don't mind for these uses. I use one of these
amazon link to turn the Warn M8 on Snowball on and off, but it's current rating isn't quite high enough for your winch. So I'd use one of these instead:
amazon link, which is rated for 600A.
If you really want a remote controlled contactor for "arming" the winch I'd suggest using a pair of the 350A rated Blue Sea Systems L series contactors bus-barred in parallel, or possibly their equal from Rincon Power if you can find a retailer. Those are hermetically sealed contactors that are rated to switch under full load roughly 10,000 times minimum, so they are not likely to ever cause a problem. It happens that I know way too much about those particular contactors......
A battery kill switch in the ground cable is my preference too. Hella makes a removable key
kill switch rated for
2500A intermittent. I like that these have a removable 'key' adding a layer of anti-theft security. They make a physically smaller switch with a removable key, but it's ratings aren't as high. Be careful of those too, they're commonly cloned in chinesium. When I put one of this type of battery switch in the Valiant as an anti-theft device I put a 2.5A ATO fuse across the switch terminals. That allowed the various computer memories to stay live with the switch off, but any significant draw, even just the starter solenoid, would pop that fuse.