I'm in sorta the same boat. The engine is not original and not quite stock. I was not surprised when my BB disappeared, and I just timed by vacuum. First the early vac advance dizzy then a 60 series big cap because all the cool kids had one. And I'd just time to best vacuum and then back it off a tad. And carry a 12mm ratcheting endwrench in the center console.
Then I got a new-fangled digital timing light and found that I was running at 33* BTDC. Even I could figure that something was wrong with my set up. So I replaced the non-functioning double diaphragm advancer unit on the dizzy with a new single diaphragm advancer from
@4Cruisers, fiddled with the advance springs for a while and chose a pair that gives full advance (according to the enclosed instructions and my SOP) at around 3K RPM. I was able to retard timing to about 20*, it's happier at 22. I pull about 16" Hg at a 600 RPM idle. I'm just over 3k elevation and this engine is 20 years old. I've been promising it valve job for 10 years. I'll get to that any day now.
I'm thinking that the timing creep is sorta normal. Don't know why, maybe the gas, maybe just age, perhaps the global warming. Maybe they ran better with more advance when brand new and the FSM is a lie.