Setting timing on a 225??

The factory sticker says 0° of timing at idle on my 70 225 automatic dart. I've got it advanced to 7-8° right now, the factory distributor clamp won't allow any more. I didn't notice much improvement if any.
That's my problem too. No more adjustment unless I pull the distributor and back it off a tooth or two or rotate the plug wires on the cap. I've got her maxed out and feel like she's begging for more. Don't even know what it's set at, I tuned to max vacuum at 800 rpm and bottomed out the bracket at 18psi vacuum. Really like to see 20-21 psi.