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I have the older style WSM which did not come with a thermometer of any type. I don't know about the mounting hole on the new model with the factory thermometer and if it is a drop in fit or not. I know you are asking about modding the new style WSM, but I'm posting this to show the size, model, install info on what I did to my old model as it will be very similar I suspect.
I added a 3" dial Tel-Tru I picked up on e-bay for around $25 right after I bought the WSM. I went with a 2.5" stem model. Dial range was 50* F to 550* F, but they make several ranges and unless you want to pay retail prices, you may have to go with a different range on a e-bay sourced thermometer. Some models can be field re-calibrated if they drift, others cannot. The GT300 I have is cannot be recalibrated in the field, but the GT300R version can. E-bay is sort of a mix, but the price is usually right. Mine was new in the box.
I used a step drill bit to drill the dome lid for the base of the thermometer. I also added a large stainless steel washer and had to drill the center hold out a little bigger as well.
That being said, I installed this in October 2005 and have had no issues with it. I do occasionally pull it to clean the buildup off the stem and check calibration.
just took the factory one off to test it out and it reads 201-202 in boiling water so it is off by about 10 degrees. also it has a 3/8 inch hole in the lid so i would have to drill the hole bigger for a tel tru. and then there is the problem that there is a locating hole so the factory threrm doesn't turn and that would be wide open.