If you are going to use the parts that you have then go ahead and use them and the power will be what it is. If you want to hit a specific power target then you need to take a different approach. Factory cast iron heads don't make over 400 hp without some help. That "help" can cost a bunch of time and money. Very few people port cast iron heads anymore so that can be a roadblock. TrickFlow 240 heads would bolt on to your short block, but having the pistons so far in the hole will kill some power and the Trick Flow heads might not be in your budget.