Casehardening on the barrel? The 36 wasn't casehardened anywhere, and especially not the barrel! If it really is casehardened then it will turn purple if it's hot blued over the hard surface. It would need to be rust blued, or annealed prior to bluing, if it's case colored. Cost would eat up a lot of bucks! Even if you got the 36 for less than $200, the cost of rust bluing or annealing and hot bluing would make the end gun more expensive than an unmolested original.
On a sidenote; are you sure it's a real ADL, or just and ADL barrel on a std. 36? Marlin sent thousands of std. 36's out the door with barrels marked "36 ADL" and most weren't checkered. If it's checkered it should be real, otherwise it's just a ADL barreled 36.