I advise extreme caution.
If your goal is to add value for sale, consider that, at least imo, the only proven "market" for w116s in general and 6.9s in particular is collectors/afficionados/enthusiasts *of vintage mercedes*. There may be buyers who like to tinker, have their own mechanical/performance goals and ideas or are otherwise wonky in the head enough to think a Panzer would do well with 53° of opposite lock wound on hammering out of a turn with smoke peeling off its (lowered, locked) rear end (and of these wierdos I hereby stake my claim as king, or at least "minor aristocrat with aspirations to the throne")... but these buyers don't want an 8hp preswapped into a 6.9. They probably don't even want the headache of a real 6.9, since an m119 swap or worse (LS? Barra? *gasp* something bmw?) will get them the go and they need something haltech-shaped to run the 8hp anyway. So they'd be more in the market for a 280/350 rolling shell in good condition, at been-on-gumtree-ages-need-gone-asap prices, since they'd want to do that 8hp swap themselves, conning themselves they can do it cheaper and better than you did. They certainly wont pay what such a swap will end up costing or more as a price premium to you. Again, hi, I'm Mark. Meet my car, and all the adventures I've had getting it to start and (briefly) run.
Look at the Motorex 2026 thread, there is one photo of a white Torana I think with an army green turbo six and exhaust pipe out the bonnet. (Engine is a barra, BTW, and the turbo size suggests "roll racing" or "dyno shootout"). The builder loves the 8hp swap, and did a lot to popularise it here. Lurking in the background of that shot I see Jason "Keep It Reet" Fearron's (sp.?) drift car "Barrawags" (R31 skyline wagon with a barra and also, iirc, 8hp). These are the sort of guys who froff over an 8hp swap. I very much doub5 they are your ideal 6.9 buyers.
By all means do the swap if you want a stout gearbox in your car, it'll give you many years of enjoyment, cope with the 6.9 torque and, if you program your tcu appropriately offer the true cake-having-while-eating experience of manual-like shifting or auto-like cruising on the fly. But it won't sell your car for much more than even a regular 450se(l) 3 spd would. Certainly less than the cost of the swap more, and maybe even nothing more.