If you are gonna be spinning the engine that fast then invest in some good rods and pistons and a get the engine balanced. AS well do the rocker stopper thing perthute said.

650 for a whole silvia is pretty awesome concidering I cant even find an engine for that price. I am Jealous

Maddat has S@#TLOADS if stuff for such a conversion and it looks pretty good quality too. If you want to retain the factory computer but still squeeze a few extra ponys from it try Kev Rowley in Brisbane. He can modify CA computers and get good reliable power so I am guessing he can do the same for SR's. He also does conversion kits a bit cheaper than maddat. If you are keen I can PM you his number. Might be worth while even just to brainstorm with the bloke.