Well, considering its like a manufacture job. I wouldn't over charge like a catering event. He is picking up at your door. I would Charge by Piece.
If they are 1.60 LB I would
Add total cost for wood or fuel+ Seasonings, plus your time (Value), MIsc Materials and meat cost.
Example Scenario
meat -1.60x 320 LBS (40 EA x 8 LBS) =$560
Fuel, Charcoal 40 LBS =$16
Seasonings 20 LBS for that much meat say your seasoning is 4.50 LB= $88
Time(just say $15 hr) say 12 hrs=$180
Total pPent is $844.00 divided by 40 Butts gives you a starting price of $ 21.10 per butt or at 6 LBS avg yield per butt is about $3.50 LB Cooked Meat. I would keep to Butt price each. Cut em a deal say $20 per butt in this situation.