I've heard that the 3.5L V6 has an antifreeze drain plug/petcock in the block. Does anybody know where it is?

 

The drain plug on the 6g74 mpi on the left bank is next to the engine mount. It is very hard to locate due to the front axle's drive shaft. On the right bank it is by the heat shield (most easily accessed from behind the front wheel). They are small bolts and the one on the right bank is sitting in the block looking like it's all by itself. My FSM doesn't have a diagram of where its located.

Hope this helps.

 

Do both need to be removed or will opening one drain the whole block?

 

Each plug drains one side of the block.

You can achieve a complete flush without pulling the block's drain plugs. Drain the coolant from the rad.. (With temp. setting on hot) fill, run, drain... 3-5 times (I use distilled water). Refill with desired percent mix, run vehicle- test mix for concentration and adjust to desired mix.