We used a tool to press them together and then installed the bolts but you don't have that.
You're installing CI baseboard for a 1600sqft house? You shouldn't underestimate the labor involved in doing this if you haven't before. Assembling sections can be difficult, maneuvering large sections can be cumbersome, and if not done right leaks will occur. Do you have the legs to support the middle of longer sections?
This is awkward, but...
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