What are you running for cylinder pressure in them L/A's?

Here's 2 examples for you. My old 360 had 160 cranking and required no less than 93 octane with 18 initial and 34 degrees total timing. My new 408 has 190~195 cranking and has no trouble running 87 octane with 22 initial and 33 degrees total. Shows you how different 2 combinations can be. The old 360 was a X headed (open chambers) engine and the new 408 runs EQ closed chambered heads (still iron) with the chambers polished and the pistons at zero deck and a .038" thick gasket for a good quench. I doubt you'll have any problems running 91.