I would mothball a few and if you have to later on scarp them or trade them off.
Prioritize your costs in the construction que and see what is costing the most to build. Put that on hold, or put low priority projects on hold.
Good luck.
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Creator of the Star Trek Mod - AST Mod - 78 Ship Sets - Conquest Mod - Atrocities Star Wars Mod - Galaxy Reborn Mod - and Subterfuge Mod.