If you land at YUL your excess booze will be assesed for Quebec liquor tax + GST + TVQ on the declared value.

According to the CBSA tariff (HERE, see the table at Appendix B at page 8) Quebec liquor taxes on spirits are charged at 124% of the declared value + 0.89$ per litre.

Once you figure out what your prorated declared value for the excess portion is you can calculate the liquor tax, and then add 7% + 7.5% to get your total.

I think you're doing the right thing in declaring the excess and will likely get the CBSA cashier's head spinning soooooo fast with this one that you'll get a stamp on your traveller's declaration card and your yellow CANPASS card and be sent merrily on your way PDQ.

Give us a full report when you get home !