The important thing with the 43l tank is it gets the weight below the spine of the bike, so regardless of actual weight, it feels lighter (assuming the same fuel load between tanks).
You'd also need to include the fittings for the big tank (two on the front of the frame & one at the rear, & the rubber damper mod frame).
My '91 R100GS thread is here
My Ride photos are here
It's just better when there's twins involved....