OK, fair enough. That'll teach me to assume
Here's an idea though.
Change the shipyard options to
Expand by <insert amount> tons
Expand to <insert amount> tons
Add slipway
Add <insert amount tons>
Retool for class
The two "expand" options use continual expansion and allow you to abort at any point. But the "Add" one requires the shipyard to be doing nothing else (including nothing building in the slips), you pay the minerals up front and lose any "unused" if you change the task (keep your existing you get to the nearest 10% completed though), but it's quicker (prob not cheaper to avoid "cheating") than the continual expand method (but they let you use the shipyard while it's going on).
Stephen