5 gallons is 80 cups. 7.4% by volume leaves you with 5.92 cups of ethanol. An additional two cups of 40% JD is 0.8 cups ethanol. 6.72 combined cups ethanol in 82 total cups is 8.2% ABV.
There. Math.
And yes, you can add hard liquor and still carbonate as long as you don't exceed the tolerance of your yeast (actual tolerance, which is not necessarily the same as listed tolerance).