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If your on a tight budget I would stick with the 516's.
With bigger exhaust valves the 516's will flow very close to the 915's in stock form.
The experts claim if you max port them, the 915's flow a little better but spending big $$$ on stock steel heads for maximum performance is not money well spent with all the aluminum head choices out there.
I have a set of 516's with larger exhaust valves on my bone stock 1966 440. With a MP .509 cam and Holley 750 DP in a heavy street car it ran high 11's.



I'd be interested in seeing the flow sheets on a set of stock 516 vs. the 915 on the intake side since 915's have better intake ports than a 516.



John, many moons ago I believe Dwyane Porter flowed and then carved up a set of 516's and 915's.
IIRC the 516's with matching 1.74 exhaust valves installed flowed close to the 915's until he started really removing material. The 516 heads had a few humps and bends in the port passages that restricted flow and couldn't be removed without ruining the heads.
Obviously once the Edelbrocks heads came out spending big $$$ to go all out modifying factory steel heads was pointless.