Building a 340 engine

As noted, European 98 Octane is about the same as US 93 octane. So we can work with that for now.

OP, with the heads you have, then you should be expecting between 350 and 400 HP.... which is the 1 to 1.1 HP per cubic inch mentioned by 273. The heads as they sit without modifications simply do not provide the flow for more HP. So I will assume you are OK with that; with a 3.73 rear gear and the lighter weight that you say, it will be very fun to drive.

For your daily driver, then the operation is going to need a wider torque curve than a race-only engine and I suspect you will not want to use a LOT of fuel. So that is indicating a pretty standard, mild performance cam. Probably a cam that is 10 degree more duration than what you now have. You can go more than that but then fuel consumption goes up and up. If you get a cam with higher lift and limited duration, then you are going to be about optimum for the street use.

Looking at a few cams, I like the Howard's 711451-08, which is optimized for the larger Mopar lifter diameter. 265 advertised duration, 220 duration at .050", 108 LSA, and 104 ICL. But the good thing about that am is that the lift rating at a 1.5 rocker ratio (like you have) is .506". That is about as good as you can do for a catalog, hydraulic flat tappet lifter cam.

So running with that type of cam advertised duration, we can finally look at pistons. The goal would be to that will work with that type of cam to produce a dynamic compression ratio (DCR) of around 8.0 at sea level there in Stockholm.

The KB107 flat top works out to a 8.6-8.7 DCR with the above cam, and that is higher than most of us can successfully run on pump fuel without detonation. If we switch to the Icon IC742, then the DCR drops down to 8.0 to 8.2 with a cam like that above. As long as you keep the DCR to around 8 or more, then the low RPM torque is going to be more than you can connect through the tires.

If you wanted more peak power, then a larger cam like the Howard's listed above would be suitable, and then the KB107's might work.