Hi Gareth,
If you are going to pull the head the best performance mods for the engine is porting and polishing the head (will also be the cheapest if you do it yourself). I'm not sure what size the valves are on the B18A but normally it's best to keep the exhaust valve the same size and increase the intake valve size if you are porting and polishing the head as this will help intake flow which is harder to do. Maybe guys with experience with the B18 will know what works.
Yeah keep the standard exhaust for now. Once past the manifold there is very little to gain in performance so I wouldn't bother with the simons exhaust. Get a local guy to make you a exhaust with as few bends as possible.
Do the cam when you do the head. Also renew the rocker assembly; new pushrods, springs, axle etc.
Brass guides for running unleaded as this help dissipate heat away from the valves.
Have a read of diy head porting:
http://65corvette.nonethewiser.net/t...al/diyport.pdf