I agree that in the overall scheme of things, you aren't talking much difference. You are getting 8.4 mpg now. I don't know what size tank an 84 Bronco has, but let's just use 23 gallon. Even if you take a 10% hit to the mileage, you are only talking going down to about 7.6 mpg. At 8.4 mpg, your range was 193 miles. At 7.6 mpg, it drops to 174 miles for a difference of 19 miles per tank. You didn't say how much you will be towing, but let's say 10000 miles per year. That is 1190 gallons of fuel at 8.4 mpg. At 7.6 mpg, it will be 1316 gallons. So if you took a 10% mileage hit, it will take you 126 gallons more for the year or about $252.00 at the current national average fuel price. (All numbers are approximate/rounded)
Realistically, I doubt you would take more than a 5% hit, if any. Personally, I'd strap it on top and not worry about it. I'd rather have the canoe to use on my trips than the potential of a little bit of extra money that I will likely blow on something else anyway.