This is a been there done that post.
I had a set of 516's that I had originally for a 383 build. I spent probably 30 novice hours grinding on 'em, then had them rebuit buy a reputable speed shop here. I then increased the valve sizes to 2.14/1.81 and the shop messaged them to get a 250 cfm flow number on the intake. They installed dual springs for the cam I was using. They worked very well and would pull well thru 6500 rpm. Your 915's would probably do a little better than the 516's with the same amount of work.
Here's the rub.
I purchased a set of CNC'd Eddy alum. Heads for about the same amount of money that I had in my 516's. These flow fully 70cfm (as actually measured) more than the maxed out 516's.
So that is why you are getting the alu. head debate added in here.
So if your going to use the 915's I say go for it. Just know that to get the full potential out of them it will cost you some coin.