A grain is equal to 64.799 milligrams, so a 240 grain bolus is equal to 15551.76 milligrams. So of your human aspirin you would be dosing 47.85 human aspirins.
(240 grains/1) (64.799 mg/1 grain)/325mg=# of aspirin needed
15551.76mg/325mg=47.85 asprin tablets
Veterinary aspirin comes in many different dosing forms including 240 and 480 grain bolus doses, meaning that it's giving all at one time. The amount of aspirin is just more concentrated in this product to allow for smaller amounts to be given to provide adequate dosing.
If you choose to use aspirin you should be aware that it does affect blood clotting, so if you are giving it regularly and your horse gets a wound he may very well bleed more than he would normally. Aspirin also has the same risks of GI ulcers as Bute and Banamine.