10.2 is WAY too high! Or could run the l28 with the E31 and a 2mm HG for 9.5........ a little better. You would have LOTS of ping and would loose power backing off the timing. There are plenty of threads on good CR look around. A l28 are not like engines today (Honda), you can't just pick a big CR just because...... You have to prep for it! Most people use methanol injection and other things to get rid of detonation. But you would be better off with timing advance than with a slight CR bump more power in the end.
Get a running engine to play around with then see where you want to go from there.
And YES early Z did run with lead. There was 5% in gas till 85..... but it will not be a problem with a higher octane rating.