GM has NOT be using freeze plugs in its newer vehicles, the 3.6 in my GXP does not have any, and neither did my 08 GT so the 3.5/3.9 do not have them either, now to cap off the bored for the oil and coolant channels they use big threaded plugs instead of freeze plugs so once the coolant freezes it can crack the block, this happend to a GXP i was looking at buying in MT when my GT got wreck from the best i could gather, as there was coolant in the oil, looked around and the block was cracked, and the block heater was still tucked away as if itd never been used and it regularly gets -40F there in the winter.....
when i put on in my GT i had to have a HUGE allen head socket and breaker bar to get the plug out and the new heater threaded into the block it cost about $30, when i put one in my GXP it just slides into a hole in the block and clips in with no coolant contact, the one for my GXP comes with a built in thermostat in the cord and the whole thing for it cost $280 from GM parts direct as its a dealer only item at the time i got mine....
im not sure how they install into the old 3.5 or 4cyl