I'm just a hack garage builder and I'm somewhat intimidated by the thoughts of building a fillet brazed frame due to concern over warpage. I've built a number of lugged frames and even those are easy to build crooked until you develop some skills and tool yourself up with a surface plate and some method to straighten the frame (cold setting). Fillet brazing takes a lot of heat and there will be warpage from everything I've read. To counteract this issue my understanding is that a specific routine is needed such as braze the joints in quarters and work in segments - don't just go around the joint all in one go. Honestly, I'm not schooled on the subject so maybe my concern is unfounded, but there is a reason so many garage builders stick with lugs.