Sounds like a Riv-nut tool. It's a special pop rivet tool that installs riv-nuts into a specific size hole. The hole diameter is sized to match your choice of riv nut, which is dictated by the size bolt you choose. In your case, it would make sense to one that matches other bottle cage bolts.

Rivnuts were actually used in many aluminum frame bikes. My Cannondale had them. Bike shops used to have this tool to repair striped or loosened rivnuts. You can buy you own tool and a rivnut assortment for 50-60 bucks.

In cromoly frames the bottle bosses are normally brazed in place. If I were doing this for a steel fork, I would choose rivnuts, because it would then be unnecessary to braze. Brazing heats the metal, and often weakens it. Rivnuts get you around this potential problem. Of course, drilling holes in the first place weakens the fork too. I think one small hole on each fork leg would be OK for mounting a front rack, but I'd leave the bottle bosses out. However I noticed the Salsa Casserole seems to be smaller diameter tapered tubing - I would leave this alone and get a whole new fork.

Consider a Civia Loring fork @ 395mm atc, 55mm rake, with all the bosses you want and disc brake only. I just installed one. Only thing I didn't like was the small Civia branding on each fork leg, but that's only a personal preference. Alfred E has the lowest price and free shipping >$75. You'll need a ROAD disc brake for the brake levers on your Salsa Casseroll Single. Avid 160mm BB7 is an excellent choice.

http://aebike.com/page.cfm?action=de...=30&SKU=FK2402

http://aebike.com/product/avid-bb7-r...r2243-qc30.htm

Be aware that going from 383atc/50mm rake fork to a 395atc/55mm rake fork will result in a wider turning radius and slower steering response. It won't be terrible, and you'll likely adjust to it after about 30 minutes of riding.