Had to remove alt as well!
Recently got the P0128 code and so at 116K decided to change the dex-cool as well as the tstat. I have a 2004 Envoy XUV and tried to go through the wheel well but as the last poster said found that the top bolt was not possible since the alt posed too much interference. Wasted some time and should have unbolted the alt from the start it would have made getting the hose clamp off easier. The old dex-cool looked great as did the radiator and block. The first step was to remove the bottom hose from the radiator and to drain the system. The block did drain through the old tstat although it was slow. I then refilled with distilled water and did this twice at which point the drain was pretty much clear water. Found that draining while the engine was hot worked best (my coolant temp was probably only 150 or less since the old tstat was opeing too soon or wasn't closing all the way. I then replaced the tstat and replaced the hoses and used screw clamps. Once the alt was removed getting to the bolts from under the wheel well worked great. I would suggest having a magnetic stick to grab the screws and I also should have worn a long sleave shirt. I was able to drain the overflow tank by removing the hose from the radiator and lowered down so that the fluid drained out and flushed with distilled water and then filled with 50/50 mix of dex-cool and distilled water. Found the dex-cool conc for $14.27 at wally world. Oh also, a short combination 15mm wrench worked good to get the bottom bolt out of the alt although I ended up using a pry bar to slightly move the ac line to get the bolt back started on reassembly.