STTally,

"Question.......The Sidereal brace/ Wing joiner is a good tight fit against the fore and aft faces of its slot but there is considerable movement in the vertical. Is this normal to allow for wing alignment or should I pack out the gaps. " Pack out the gaps but allow enough gap so that the epoxy can fill the joiner.

Is epoxy best for the wing joining job. Yes. Use 30 minute - gives you more time and is stronger than 6 minute.

"The marks on the firewall for the engine bearers are pre marked but off set....is this to allow for engine downthrust / sidethrust....or is it just poor marking out. "
Probably for down thrust. I would temporarily mount the motor using a couple of dabs of CA and the install the cowl with tape. Looking at the end, the motor shaft should be centered. That should be an easy check.

Later,
John