I just finished cutting the top (well, extending the cut) out of a 50L keg, and i can say, the most ACCURATE thing i used was the Dremel with the metal cutting blades, just rest the blades on the steel, hit it to 10,000RPM and move it along your line carefully. Took about half an hour to go around the entire top.
I also would recommend a step bit if you are drilling, go for a pilot hole, and make sure you keep it cool, lube the cutting parts with wd-40 or something and then just use water, otherwise you will end up with an oily keg that you have to wash down, i did one with WD and the second with water.
If you arent cutting the lid, i'd just go a grinder with the Steel cutting disc, straight lines are pretty easy, just go slow, and spread the cut wide so you arent in one spot too long.
Wear a mask, goggles, gloves and some clothes that can get dirty, the steel shavings fly everywhere!
I'd do a keg rotisserie!