273 Can't get too much timing - confused

If the ground wire is clean to the shell you can bet you have a good bit of timing on it.

You don't always hear detonation.

You may have enough cam and low enough compression ratio that it won't detonate until the timing is high enough it puts the starter in the cheap seats.

I have a Pontiac 400 that's the same way. It has 46 total on it now and I can't make it rattle. I suspect the damper has moved, but I'm not even about to verify TDC. That turd need to come apart and be rebuilt.