Building a 340 engine

To really know what you can safely overbore, to, you really need to sonic test. Some 340 blocks are on the thick size and will easily go .060 over, or 4.1 inch bore size and have metal left (340 T/A blocks in particular). How much is 98 octane gasoline a liter there in Sweden? Here in the U.S., our pump octane rating is an average of research octane numbers and motor octane numbers. Are your octane numbers RON, MON, or R+M/2 ratings? More than likely, it is RON and in reality equal to 93-94 octane at the pump. If you have a MON rating of 98, you are probably going to be very close to R+N/2 of 104 rating.
I also notice you listed the Edelbrock magnum type head with the 58 cc chamber. An example for you if you leave the Edelbrocks at 65cc's and use the following Cometic gasket.
Cometic MLS Head Gaskets C5917-036
If you leave the piston .002 below deck to have .038 piston to head and use the 4.06 bore piston with 5 cc valve notches you come up with 12.3:1 final compression. That's okay if you have an actual R+M/2 octane rating of 104, and might give you a pretty fair shot at 500hp with the right cam.