Crank end play, destroyed converter, 408 nightmare

All the CR numbers make sense... ran them this AM and they all hang together with the latest info from the OP.

98 RON (research octane number) is indeed about the same as 93 octane in the US, which is the average of RON and MON (motor octane number), and gives usually about 5 points numerical lower results.

IMHO, 10.5 on 91 US octane (the 95 RON in Europe) may be marginal. But I also think the final SCR may be lower... the OP has not told ups if the deck of the engine block was cut to reach true zero deck for the pistons. With a nominal 360 block, these pistons are around .010-012" in the hole. So actual SCR with the .086 thick head gaskets may be more like 10.1-10.2.

Additionally, most of Austria is at or above 1000' altitude, and much higher in much of the country. That is going to lower the operating compression ratio. From direct experience, going from sea level (where I think autoxcuda lives) to 1000' altitude WILL make a very definite improvement in detonation resistance.

OP, you may want to increase the head gasket bore hole to 4.10 or 4.12. The gasket cylinder bore holes do not always perfectly register (align) with the block holes so some extra opening size is wise.