Jack, don't forget the QRP-LABS kits can be used with a GPS receiver to calibrate the Si5351a chip. I had been thinking about using the Si5351A with my Heathkit SB-101 but it seems you guys beat me to it! so things I want to explore myself when the time comes.
1) sweeping the filters to find the sweet spots, so adjustments can be made to all the oscillators for peak performance. (this might even be employed per filter as tweeking the VFO,BFO, and heterodyne settings should be trivial)
2) trying low side injection to see if maybe 4 meters or perhaps 6 meters can be reached.
3) designing and laying out a new audio board with a Class D solid state driver stage for the speaker and replacing the combination triode/pentode tube with something a little easier to find.