Do you really need a H-box? If you check recent threads most will tell you they seem to do little.....(assuming you are using baffled mufflers/silencers)
If not wanting the original look, possibly just separate pipes?
If definitely wanting a H-box, then remember this is the part that often rots.....so maybe some mods b4 fitting?
AFAIK the only difference between the GL650 and the GL700 is the badge. From what I gather they were called 700s in Japan to make them sound like the biggest bike you could buy there at the time and 650s in the rest of the world partly so buyers would pay less for insurance and partly so they would compete with 650s in the marketplace instead of the strangled 700cc versions of models designed as 750s that US tariffs demanded at the time. Both have the same 673cc engine....