I agree with your thoughts Yves. Maybe I'd do compression and leak down tests just as routine checks, for peace of mind. And I'd service things; tune up, etc. I do not think the engine temps you mentioned are anywhere near too cold. In fact that was considered high temp back in the era these cars were first designed. I've never experimented with any multiple electrode plugs like you have. I have found (generally speaking) that some engines seem to do much better with certain plugs overs others, and vice versa. As peculiar as it may sound, perhaps the SOHC does not do as well with those multi electrodes types? Also check over the rest of your ignition system just to make sure everything is functioning properly. Same with the EFI system. The comments about injector servicing reminded me of a very interesting discussion I recently had with the engineer from "GB", the company that rebuilds fuel injectors. I'll add something to my thread about the SEMA Show in the NFC section to relay what was said.
I can't resist; what brand of oil are you using? Only joking...my comment about the plugs was bad enough, lets not start one of those debates again.