You would have better results as far as inflammation reduction using Bute, however Asprin will also reduce inflammation and will not "mask" inflammation for days as Bute will. Merck Veterinary Manual
There is some more information. It seems 25mg/kg is the accepted therapeutic dose. For 1000lb horse that is just over 11,000mg (assuming this is once a day).
If you have 500mg tablets that is about 22 tablets per dose.
My mum used Asprin on her arthritic horse and administered it about 30 minutes prior to riding. Because it is not long acting it was good for keeping him comfy during rides. She used the vet prescribed large tablets that were about 4000mg per tablet and she did 2-3 per dose, if memory serves.