Well 40lbs of pork butts is 4-5 butts correct? You should be able to arrange them around the rack and then put the grate probe towards the middle of the grates. As long as the meat isn't overlapping you will be fine (if they touch a little bit that is ok because they will shrink)
If you want to ensure that the butts are done by 10am, I would put them on between 10-11pm and cook at 275 degrees. This should get them done in about 1 hour per pound and give you time to.
The way I would prep the uds is to dump about 3/4 chimney into the uds. Once it reaches 225 I would close off 1 vent. At 275 close off the 2nd vent. at 325 put the meat onto the smoker. The temperature will take a while to go up because of the large amount of cold meat you are putting into the cooker. Adjust your vents accordingly to maintain the 275 degrees.
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