There's different causes sometimes, even if they sound similar. Too hot a plug can cause knock. So they call that pre-ignition if I remember right.

The thing about excessive vacuum advance ignition timing, it shows itself in a different set of conditions than with excessive mechanical advance, which is indicated when under heavy acceleration as from a dead stop or anytime high demand & high RPM. A rattle or ping there is too much initial timing, or somewhere later in the mechanical curve based on engine RPM. Don't want that, ever. That's why it's important to suss out the mechanical advance with test drives while vacuum advance is disconnected and plugged. Get the mechanical curve dialed in where you want it, then you don't want to mess with it again, and tune the vacuum can only. Where vacuum advance really earns its keep is when cruising on level ground, in high gear, engine load is low, and so is RPM. So the mechanical advance is relatively retarded. Here the vacuum advance can pull in a lot of timing for smoother, cooler running and better fuel economy. Fuel mixtures are very lean and therefore need a lot more advance to get the fire lit.

Vacuum advance ping or knock is a little different in terms of where it's found, the split second rattle sometimes heard at part throttle acceleration when cruising or brief no load deceleration just when letting off the gas. It's based on engine load only, so a very slight momentary rattle every now and then at part throttle is probably OK. If excessive vacuum advance is pulled in though, it will be a constant rattle and run ragged at steady cruise on level ground esp. once load levels off, confirmed by going away when accelerating (load) Not good either. So you want to be mindful to distinguish between the two.

If you don't have a knock sensor though, be careful. The spark plugs should be inspected carefully. With a magnifying glass, look for evidence of tiny round globules of silvery metal on the porcelain. Those would be the pistons self-destructing. If you can get 36° or 38° BTDC (without vacuum advance) and without knock maybe try bringing it in earlier, with lighter springs, by say 2500 and see what happens. Heavy trucks, or hauling heavy loads, wouldn't like that too much generally. Or, try a little more advance, every engine is slightly different. But it's true, engine knock is not always audible, and you want to leave a little on the table to account for temperature and humidity changes, or a bad bag of gas.

One question while I'm thinking of it - have you verified the mechanical advance numbers with a timing light on the damper?

That's a really good point. Advanced ignition timing alone will do it, along with leaner fuel mixtures when fine tuning, installing hotter ignition coils after ditching the older style points and condenser deleting the ballast wire. Factory plugs tended to be a bit on the hot side anyway to help prevent fouling due to short trips and around town driving. Extended summertime highway trips may benefit another step down temporarily. The worst that can happen with colder plugs is fouling, so when tuning starting with a colder plug and working back up if required is a good strategy. This is one reason I like NGK plugs because they have a wide selection in heat ranges. Other plug lines aren't so good about that, and cross-reference charts aren't really particularly accurate. "This will fit" is about as close as they get sometimes.

Are you certain the vacuum advance is working? Or, are you sure it was maxed out when you started? It will add quite a bit when checking advance in neutral, there is no load at all, so it will pull more in on the damper than it might out on the highway to some... degree. So you should see around 50° BTDC with a light at high RPM with the vacuum advance connected. Somewhere between 40° and 50° when cruising on level ground is stock distributor curve. Whenever accelerating under load the vacuum advance gets pulled and timing reverts to just the mechanical + initial. Sure sounds like you're on the right track. Kind of a hassle, but it's the only way to do it without a distributor machine.

On the Ford distributor, lighter weight springs can be installed, but, there is also an access slot on the advance plate, it allows one to tweak or bend the spring arms with a screwdriver without removing or disassembling the distributor. By stretching any given spring tighter it slows the rate of advance, loosening it allows for quicker advance. Notice there is often a light spring and a heavy spring, the heavy spring being relatively loose. Loosening the light spring will bring the timing in earlier/faster. Or try installing two light springs. Keeping in mind the heavier the vehicle, the less ignition advance it will tolerate.

It really is a cut and try thing. It may be your distributor has something similar, to allow for looser springs regardless of how heavy or light they may be.