Engine/Exhaust Issues

I have a 1999 GMC Sierra with the 4.3L V6 engine. While driving today the low coolant warning light came on. I carry extra 50/50 dexcool coolant in my toolbox, so I pulled over and investigated the issue. The coolant level was slightly below the hot line and the vehicle had been running only a few minutes. I added some fluid to the reservoir and checked the warning light. The light went off but I proceeded back home anyway. After this occurred I noticed white smoke coming from my exhaust while at idle and it dissapated as the rpms increased. After noticing this, I remembered that when starting the vehicle for the first time this morning there was some white smoke, however I dismissed this as steam considering it was cold outside. When I returned home (barely, as the engine was running very rough at low rpms and almost cut out as pulling into the driveway) I shut off the engine. I waited approximately 5 - 10 mins and checked the level of the coolant as well as the engine oil. The oil was in good condition (not gritty or black) and the coolant level was okay, however I did notice a leak under the vehicle which I investigated and determined to be coolant. I attempted to start the vehicle again and the engine would not crank over. After 4 - 5 attempts the engine finally turned over very harshly with a loud clanking noise and the engine started. Once started there was a lot of white smoke coming from the exhaust and the smell of burning coolant (I am pretty sure). After noticing that it was running very rough and producing a lot of white smoke from the exhaust I turned off the engine. Once off, the white smoke from the exhaust continued to come out for a few minutes (5 - 7). I am thinking that I may have a cracked head gasket, but am looking for other opinions and solutions to the possible problem. Thank you for any help and sorry for the long description but I wanted to be as detailed as possible.