I have a 99 suburban with a 350. It is burning oil at the rate of about 2quartsevery 3k. Also, I poured a gallon of Antifreeze in the other day. I haven't really kept track of the water, but I'm guessing that its using a gallon every 7.5 to 10k. My first thought was a head gasket. However, someone else mentioned it might be a intake manifold gasket.

 

Go with your first theory.

 

the intake manifold gasket is known to leak i just did mine at 130k