Phil,

Don't glue in your spreaders.
Get the right length on a spreader, then put a thin coat glue from a hot glue gun on the end surface.
It has lots of friction, your hull sides will be substantially vertical and you can just wedge the spreader in and it will stay well. String to pull it out will be good, but not required.
I glued in spreaders on a boat with yellow glue (I thought it would not hold strongly) by the time I got around to taking them out they were well attached. I used a chisel on a pole and punched a hole thru the hull up front. I finally left one spreader in the bow to avoid doing more damage.

If you have a deck that does not roll down to the side (relatively flat or a 90 degree joint) you can just grind off as much of the deck as required.

You know I finally figured out that the hull is very close to correct and the deck spread out - or flattened which made it wider. Not quite sure how this helps me on the next one, but I'll do something different!

Good luck.