There are plenty of stock block cars pushing way over 500 horsepower, its all in the tune as Steve said, keep a safe A/F ratio and keep it dialed in, get a a standalone to further watch whats going on with everything. And dont O ring your heads, go with some cometics for your bore, and some ARP headstuds, you can easily push 600 on a stock block, I can think of at least 8 guys I knwo who are doing this.