Rebuild 915 heads or go aluminum?
These days it costs a small fortune to find a good set of cast iron cores and to have them fully rebuilt. Getting cast iron heads ported so they make decent power is going to cost a whole bunch more. The only customers that I've seen spend money on cast iron heads are the resto guys, or the NHRA stock eliminator guys.
20 years ago I tried to build a super nice set of 906 heads. By the time I was finished I had more money invested in them than what a set of aluminum heads cost. And the cast iron heads were still down on power. I haven't messed with cast iron heads since.