This is exactly the same thing i have going on with mine. Same gap. Same spot. Im not sure what step I screwed up. However.. I went online to see some perfect builds and compare the engine compartment to my own to look at positioning stuff.
I noticed that the engine cooling tank.. which mounts directly behind the propeller. The part that connects with the two pipes. On the builds that look absolutey perfect and almost seemless.. those are closer to the valve cover. Almost flush against them. Mine.. and yours has a substantial gap. However. I'm sure while you were investigating you noticed the same thing. There's no possible way to move that tank back any more than I have it now. I even pulled mine and tried to see if there was something I missed.
I'm at a loss. I just put it all back together and I'm considering that I may just have to live with it. I've shown everyone my rough mock up and everyone says it looks good. But.. you know what it means to be a modeler. There's always some little thing that you feel didn't go well and now it's all you see when you look at that model.
I think my answer is going to be to paint the exposed sections the same camo so it at least doesn't just look obviously cockpit green. It will more or less blend in.
Honestly, I began to have difficulties with that entire frame. The one that attaches the headers. It wasn't just straight forward and I should have stopped at that point and found what was causing so many issues. But.. it may have been too late even then.
Hard lesson learned. I want another though! I feel like my next attempt at this plane would be phenominal!!