Very Mild 318 Build

Compression is not the enemy;
too much EFFECTIVE Cylinder pressure for the gas to be used is.
Wiki says you are at 600ft or so. You could stand a lil more cranking cylinder pressure (CP), no problem.
CP is the result of Static Compression Ratio (SCR), and the Intake valve Closing Angle (Ica). The best way to increase your pressure is with a high-compression piston..... but they are getting harder to find.
EFFECTIVE Cylinder Pressure (ECP), is determined by how much air the engine is ingesting. If you had a wee tiny one-barrel on your 318, you could run a lot of CP, cuz the engine would never physically be able to pull enough air in, to make a problem.
Another thing that comes into play is engine loading.
If you have a tall rear gear, like 2.76s, then 60 mph is about 3400 rpm in 2nd gear, so, the engine in second gear, is never gonna have to work hard, never get up on the cam, and never make a lotta EFFECTIVE cylinder pressure. So,again, you can run more cranking pressure.
If you build a low-powered engine, then you have to worry a lil about first gear. Again, with 2.76s, 3400 is now 35 mph, and this gear ratio will allow you to hit 4800@50mph. So now because the engine is low-powered,going from 35 to 50 is gonna take a while, so the engine will be working "hard". In your case, not to worry, because your engine would be lucky to hit 65/70% VE (Volumetric Efficiency), so it will still not be ingesting much air. My math says 440cfm@4800 @100% ,so 310cfm at 70%VE, So, again, you can run a good amount of cranking cylinder pressure.

Your engine only requires hi octane gas, when the combustion pressure in the chamber, gets to be high enough to cause a temperature rise, that causes detonation with a lower octane gas. Cylinder pressure that you measure with a gauge is only a part of this. And this detonation usually is directly related to how much air is being ingested. So if you had a cruiser that never saw WOT (Wide Open Throttle), and never sees over say 3600 rpm, for example, then you could run a lot of cranking cylinder pressure, because the EFFECTIVE pressure would always be low enough to burn cheap gas.
So then, you could run 87 all week, and then, on Friday, anticipating some romping on Saturday, you could tank up with fresh hi-test, and then let her buck.
The point is not to be afraid of SCR.
In today's engineering SCR means very little. Today, we use SCR to change DCR (Dynamic Compression Ratio), which is actually what the engine is seeing. We try to get the DCR , up into a zone that we know will be adequate for the intended useage.
DCR and SCR are cousins, the only difference being where the piston is at the start of the math. Scr uses the piston at the bottom of the stroke, and DCR uses the Intake Closing Angle. Neither of these care about how far the throttle is opened, but with Dcr , if one assumes the engine is not throttled nor exhaust choked,then one can assume a certain cylinder pressure working range for the intended fuel to be used.
Like; with open-chamber iron heads at sealevel, the upper cranking pressure limit, for WOT operation, is about;
160 for 91
155 for 89
150 for 87
The Effective cylinder pressure could be fractions of that, always less, and depends on the throttle opening, and rpm. At idle, for instance, the running cylinder pressure might be, I'm guessing, just 30 psi...... so you don't need even 87 gas for that. Running the primaries WOT (on a 150psi engine) will not even require 87 gas. The only time that engine might require 87 gas is with All 4 bbls open,and at going thru the torque-peak,and towards the power-peak a ways, where the engine is gonna be most Volumetrically Efficient, VE.
VE is simply the ratio of what volume of air the engine can ingest compared to it's volumetric displacement. For a 318, 100% VE is ~39.8 cubic inches per cylinder.
in conclusion;
Compression is not the enemy; to much EFFECTIVE Cylinder pressure for the gas to be used, is. With un-milled open-chamber heads I think it would be very hard to build too much pressure in a 318LA. and easy enough to drop some pressure with a lil cam and it's later closing intake angle.