As others suggest, I'd also say you need at least a 200+ hp size tractor to handle 40' bar. The weight of the applicator (mounted I presume) will make the front end light unless you can get a lot of weight up there. Transport isn't safe with a smaller tractor. If that big bar starts waving around while roading, you can find yourself heading for the ditch.

I'm sure someone here pulls a 15 knife with an 8N Ford just fine, but our experience says get a bigger tractor, or drop to 12 rows (11 knives). Don't forget, you will also be pulling an 8-10,000# tank behind the applicator. If it ever gets a little soft, you'll be looking for more tractor.