As far as I can recall, the gearbox is held onto the frame angles below it by long bolts that go right through, and have presumably come loose - or maybe one is missing?
The front chains from the gearbox are tensioned by moving the whole engine/gearbox assembly along with a long bolt that goes through the end of the sub-frame - it's ages since I grovelled inside mine and I can't remember if it's at the front or the rear of the assembly - I will have a peer inside mine tomorrow, if I get a moment/remember!
Adjusting that doesn't affect the axle-to-axle chains, they have separate tensioners, and the axles cannot move along anyway.
Rather than splitting the hull, I would take the bulkhead out for better access - mine is pop-rivetted in.
Hope this helps,
John