I thought I might tackle your question because it is baffling. I had to think for a moment to see if there were anywhere for oil to disappear to in a 351 Windsor. If it isn't leaking it has to burn it; period (my car does this as a habit). Just because it does not smoke does not mean it isn't burning oil. I thought about it for a short time and came up with one solution only. Your motor has less than 5k on it which means that you had it re-worked in some manner. I would hope you had it rehoned when this was done, but I don't know. If you did, this will eventually seat the piston rings for a better seal, but this may take some time and may use more oil than a broken-in engine until the rings wear-in. When rings wear in, they deposit metallic residue in the oil making it black. My suggestion is to monitor the oil use for a time and if it still uses oil, perhaps the rings weren't installed right or worse. The oil control ring is especially important and the 351 to 420 displacement tells me the bore is different. What I have found is that a 351 stroker kit with a 4.00 throw makes a 408 not a 420, with a .030 over-bore. If someone used the wrong rings i.e. a .010, .020, then you would have blowby and use oil. This is the worse case scenario. If your engine continues to use oil steadily without any improvement, i would tear it down and measure the ring diameter using a feeler gauge inserted for the ring gap. If it is .010 or more smaller than 4.030 bore then someone didn't put the right rings in your motor, probably because they did not check them when they pulled them from the box. If you find this is the case, make sure you check all three rings the same way. Check the compression also before a teardown for a quick check to see if your compression is up to par. A good pressure should read 120-160 psi during cranking (make sure you disconnect the ignition)LOL.

some stroker kits have the oil ring going through the hole for the wrist pin, this allows a small amount of oil to pass behind the ring and to be burnt. The piston on the left here is what im talking about: http://www.moddedmustangs.com/forums/files/p40345_image_small_978.jpg If this is the case, there is nothing you can do but use a different piston that is not like this.