At the Butterhouse Ranch, you'll find an outhouse saying "Curly Was Here" telling you to go to Kole Ridge Mine to find something marked with the X
Find Curly compass by digging near the three trianges at Kole Ridge Mine.
Follow Curly's compass (just go the direction it tells you to unless the message changes and it gives you the option to just follow the compass -- I think this always takes you to Alexandria's Ranch, but I could be wrong). This requires you to have access to the areas of the map post-Breadwood. Check the compass when you get where you need to go (Alexandria's Ranch, I think). It will take you to Curly's cairn where you'll find Curly's Auto-Gyrotheodolyte.
Take Curly's Auto-Gyrotheodolyte to Madness Maw Mine. You'll see once you enter if you check the device, the lights start going off. Eventually, near a spiral on the 3rd level, you'll have the option to dig a tunnel, revealing Curly's Cave.
Once you loot the Cave, you'll get some items and half a treasure map. Examine the map. The other half is with Halloway.
You'll eventually find Halloway at his hideaway (Halloway's Hideaway) in the third part of the game (once you've reached Frisco).
He'll give you the other half of the map if he trusts you. You can gain his trust by giving him some of his stuff. His pin is in a locked footlocker in the bunkhouse in Breadwood. Examine the map.
Once the map is complete it'll reveal the X outside Halloway's Hideaway. This gives you Curly's Strongbox which gives you a ton of meat.