273 Can't get too much timing - confused

Checked TDC with a piston stop. After rebuilt distributor I started with 16, then bumped it up to 20 when it didn't ping or kick back the starter.

Then after a 70 mile round trip to work, I pulled some plugs and see the entire ground strap is clean. Runs great! Much better with the rebuilt distributor.

I'm just concerned about getting detonation. Wish I had a scope to see the pistons. Think I'll back it off a couple degrees.