On of the engines that I used the larger valves in was a 261 that received a larger cam, 390 Holley, and Fenton exhaust manifolds.
I do not know how much of an increase the valves accounted for.
I think Jerry is right on this.
I also have used the larger valves on heads with a lot of pitting in the seats or seats that have been ground enough that the valves are sunk in the head. This was done instead of installing hard seats.
