I think the 4.3s were more sensitive to detonation judging by what I see in my OMC shop manual, the V8s both can run with more total advance than the 4.3 which is more limited:
4.3: base timing 6* BTDC, total advance at 3200 rpm = 18* (6 base + 12 mechanical)
5.0: base timing 10* BTDC, total advance at 3200 rpm = 22* (10 base, 12 mechanical)
5.7: base timing 8* BTDC, total advance at 3000 rpm = 29* (8 base, 21 mechanical)
BTW the gas pumps we have at the 2 gas docks close by only sell 93 octane gas anyway, but in my shop manual it indicated if your are running lower octane gas the 4.3 should be timed at 1* BTDC.
Before doing anything to your base timing you should use an advance timing light to check your full advance with the boat in the water so you can run it up to whatever the full advance spec is (if you can't find it use 3200-3500 rpm) boat anchored or tied up lol!
You do not want to get into a situation where you have detonation but due to the noise inherent in running a boat you never hear it before its too late!
What ignition do you have (Delco EST?) and is this carb or throttle body injection? Pay close attention to the color of the center electrode insulator of the plugs....you don't want it to be blistered white.....light tan or grey is good.....