A Mighty Vac tool is very useful for not only operating vacuum advance, but has many other uses on these trucks (testing hvac components, cruise control,even bleeding brakes). Good luck
I work at a Caterpillar and Cummins dealer and our parts availability is hit or miss as well. Lots of stuff is USA sourced, but come from states that are shutdown. I don't think this is going to improve until people get back to work...
Chris, if that's a factory bracket and is readily available, I may notch the rear outboard fastener boss a little bit at a time while trial fitting the valve cover. Then maybe fab some type brace connecting to the forward most header bolt. Good luck