New XT600 owner here starting with a pretty decent '86 43F. My question is how easy is it to convert to a clone 34L/55W? Are the tanks interchangeable? Do you have to do the seat and the tank to get the "big tank" look?

TIA

Quick reply.
Everything is possible given enough time, money and expertise.
From memory the side panel locators are in different places and the tanks held on differently. If you can fabricate the mountings you can do it
The short seat on a 34L has different rear mounts as well.
The biggest problem will be finding a good tank and side panels.
As is always the case you'll be best off finding yourself a nice 34L, 55W or 1VJ to buy if you want the big tank look, or finding yourself an old Acerbis tank to fit your 43F

Bad.
Save the money and put it towards a Tenere.
Keep both bikes, they ride quite differently. Each has its merits.
(Comparing 43F with 1VJ.)

As far as I know the frame of the 43F is way shorter than the Tenere frame, so the tank would not fit.

Andy