Hey Doug thanks!
The whole superstructure swap thing works well! You'll see from the below picture the flat deck of the HMLS Swiftsure ready to be slid into place...
These are a couple of old pictures showing how the deck fits into place. See in the example below with a bit of white stryene substituting for the superstructure it fits in the top of the hull and the two arrows show the way it will slide into place...
After sliding the superstructure forward it is 'locked' into place, you'll see in the picture below in the example the left arrowhead is now hidden in a gap in the hull so it won't fall off...
Cheers!