Jay, are you familiar with bilaminate construction? Claude Butler made frames this way in England after WWII when there was a scarcity of materials. With this method, the sleeves are not make into a lug 1st but rather the sleeves and tubes are fillet brazed together at the same time. I prefer brazing a sleeve to the tube 1st and mitering them as one unit. This way you don't have as many problems with distortions.

The proper slip fit sleeve is 1/8" bigger than the tube it is being attached to with a wall thickness of .058" You can buy this tubing at Wicks or Aircraft Spruce. However an 1/8" is pretty thick so it is desirable to turn the outside down so the wall thickness is closer to .040.