It's funny you mentioned the lip as on mine that has been bent down and gives a 20-30mm gap between the back of the cylinder head and the bulkhead.

When I've changed the box (as I'm assuming most do) I have to tilt the engine and gearbox right down in order to separate the box from the engine block. I also always have to drop the ARB to give me more room. Mine however is a fully fabricated conversion with the tunnel and all mounts made from scratch.

You mentioning the lip has now made me realise why perhaps mine has been bent down - giving more room. So if this is the only difference on yours to a standard panel I would probably say that's it. I'm guessing the tunnels have a fair amount of space around them normally which is why you don't usually have to drop the ARB?!