Hi Jon, If you look at the thread "650 engine questions" (The 650 cam has a duration on 263 degrees (depending on year). A 30/70 cam has a duration of 280 degrees, a 35/75 cam has a duration of 290 degrees and a 40/80 a duration of 300 degrees.) you should get an idea of the differing cam profiles. If you are rebuilding your engine it will take little extra time and cost to 'relieve' the porting in your head to allow the engine to make the best of the larger valves. However, as in all engine upgrades no matter how small don't go mad in only one area as improving the breathing will put extra stress onto the bottom end and without checking can give horrendous problems down the line. Carefully check the wear on your big ends and main bearings and regrind if in doubt. skimming the head a little will also improve engine response and you should not encounter any extra worries that come with radical mods. The carb may need rejetting slightly for good running, but that is simple to adjust. the engine is a jewel and responds to gentle modding and you will find that you will be able to keep up with the modern stuff on our roads!!!! Any questions please ask.
Ian.