To quote Tech,its all about the combination.
Yes you can buy/build better heads.There are many choices.If you know exactly what you want,then build that. The 350 you have can produce 500 plus HP in street trim. If you need more,then consider a bigger engine. If you use a 421,then 500 horse power is mild.If you you a 355,then 500 horse power is getting more difficult to deal with on a daily driver.