Given that Richard's comments are emminantly true, there's no substitute for cubic inches.

However, if you're absolutely married to the 305 you need to look at World Products heads. These guys are the kings of Chev heads (let the arguments begin!) and their prices aren't too far north of high quality rebuilding costs for stock heads. Check this link: http://www.sallee-chevrolet.com/Worl...uer_Heads.html

Here's a link to a synopsis of how Car Craft built a 325hp 305: http://www.goingfaster.com/spo/325_h...chevrolet.html