I'm no cosworth expert but if you were going to go in stages you'd be best starting with higher spec internals IE steel rods, forged pistons etc.
The difference in 300bhp and 450 bhp would require different components. I would think that cam choice, compression ratio and the like would have to be specced to what your desired output would be.
I would have a go if your confident as you have plenty of people no here on hand to give good advice plus if you look in technical sections on here there are some cracking posts.
Good luck and let us know how you get on.