The fuel you can run is directly related to one thing only,detonation.
Detonation can be avoided by two or three or four things.
The major players, in the building stage are
Combustion chamber temperature, and chamber efficiency.
The former is mostly related to cylinder pressure, which is in some part affected by static compression ratio, but also by the camshaft's Intake closing angle, followed by inlet air temperature, AFR, and coolant temperature.
The latter by head material, combustion chamber size and shape, quench,freedom from hotspots, ignition timing, and loading.
I think I got that right.lol.
As to pressure, for 87 gas you are sorta limited to 150/155 psi with iron open chamber heads. But there are many many ways to achieve that pressure, and Static Compression Ratio is just a tool we use to get what we want.
It is easily possible to run 11/1 on 87E10; I have been doing it since 1999, allbeit in an aluminum-headed 367. Even ran 11.3 with a MP-292/292/108 cam.
Depending on the size and weight of the boat,and the rear gearing, which goes to loading; and the cam used, you may not have to lose a meager 4/10 of a point.
Furthermore, if you have to pay 10% more for the next grade of fuel, but you build your engine to get 10% more miles per gallon; then by the math you break even. But in the real world, the additional cylinder pressure allowed by the better gas, will end up saving you additional fuel during acceleration.
It's a lil tricky, but well worth the effort.
As an example
@9.4Scr, you can get about 155psi@ an Ica of 63* on your cam,which is a nice sized cam for a 408.
@9.1Scr, you can get the same pressure at an Ica of 58*, which is not quite two sizes smaller.
You can push the pressure up a bit by reducing your power-timing in the lower rpms, where, with a stroker, you won't need it anyway. Then you can bring the timing back with Vacuum advance, for cruising with. And you can still have full power-timing after say 3600 for passing.
But, if you have an overdrive, and you intend to tow in it, all bets are off. With a non-computerized ignition timing, it will likely be impossible to get the timing up high enough, and your fuel economy will be in the dumpster because of it.
You see, everything we do in terms of timing controls is designed to do one thing; namely to time the spark in such a way that the maximum amount of energy in the expanding hot gasses,can be delivered to the piston, when it is on the exact right spot to deliver said energy to the crank. If your timing is too early, you lose power and risk detonation. If too late, the fire chases after the piston and you lose power and increase the running temperature. Cruising at 60/65 mph towing a boat, in overdrive, is not the time to be giving up power or overheating. Less power requires more throttle opening to get what you need..... and there goes your fuel-economy. The problem at low rpm/ small throttle opening, is to give the engine the timing it is gonna want. It is not uncommon for a lightly loaded engine to want,at say 2400rpm, 44 to 54 degrees of advance. Say yours likes 50*. You can get that with 22 in the Vcan and 28 in the mechanical. But if 28* at 2400 causes detonation at WOT, well you can't have that. So if by testing you find your engine does not want more than 24* at 2400 at WOT, then your cruise timing can only be 46. Which is likely to be 4 to 8 or more degrees short of where it might need to be for best fuel-economy.
So to marry these, the cylinder pressure would need to rise up, so that it doesn't need the extra cruise-timing.
The take-away here is to not try to cruise at too low an rpm, thinking that you will get better fuel-economy. The potential is definitely there. But if you bias your distributor for cruise-timing, you will very likely get into trouble with your FULL-LOAD WOT PowerTiming. You might be able to overcome this with a smaller than usual carburator. Or a two-step timing curve, which is what I did. For instance;My D has a fast curve to 2800, then it takes another 600 for the rest of it to come in. My engine doesn't get full Power-Timing, until ~3400.