I am thoroughly enjoying your treatment of the Tank. What a treat to observe!
I have the same boxing here. Watching your struggles motivated me to start trial fitting much of kit in an attempt to find the root of the evils which befell you.
Without coming across as a smart aleck, I believe I've found the cause, and a fix. What seems to be going on is tolerance stacking.
The wing spar out board of the engine pickup mounts appears to be approximately 1.5 to 2mm too long ( .75 to 1mm each side).This gives you the gaps you experienced.
The fix is to saw through the spar and re-glue. The kerf of about .7 mm each side results in a noticeabley better fit on all the other joins.
In addition to taking .7mm a side from the front spar you will need to remove the same from the rear spar pick ups which project from under the cockpit.
I realise this is cold comfort to you (unless you're building another tank), but perhaps others might find it useful.