Firstly I think you need to be congratulated on your post, considering english wouldn't be your first language! :jamie:
We have many aussie members who stuggle to write such a coherant post/thread.
Anyway - Being a torquey, long stroke motor there's not a whole lot that can be done about the revs. Yes sure you can put an aftermarket camshaft in it, but this would want to be pretty mild - which will keep you some decent torque still too.
I can help with some of your other questions - Yes a 3/4/5k DCOE manifold will fit the 7k. They even have the same sized ports, so you wouldn't have to do any port matching. Same with extractors. Distributor - Is the stock 7kc one electronic (it should be since 5k's had them)? If so just use that but get it re-curved to suit. Probably just easiest to leave the CR stock, or you could plane the head a bit to up it, but keep in mind your hydro lifters when you do. To keep it cool, just recore the stock radiator to a bigger one if required, and maybe fit a thermo fan and thermostat switch. Oh and most of the rules for bulding a tough 4/5k would still apply to the 7k :wink: