What's bad about 15s? The chamber volume is quite desirable for a 406 intended for 93 octane. Mill them .020" and you have 86! You must "CC" them, of course, but I just did a pair this week, and they measured 90.
15s have small valves, too. This makes them GREAT candidates for porting and making good power. By using modern "profiles" on the valve seats when they're "opened" to 2.11/1.77, significant flow and efficiency is realized over factory "large valve" heads. Some mild porting and you can have a really good pair of performance heads.
There are two potential "downfalls". They have pressed-in rocker studs. They should be reomved and the holes "HeliCoiled" to 7/16-14. The existing hole is exactly "right" for the HeliCoil, and those threads are stronger than those that would be tapped directly into the iron. The other issue is the outboard exhaust manifold holes aren't drilled and tapped. Unlike the later 7K3s etc., the "boss" IS there, so it's trivial for a machinist to properly drill and tap them for your headers.
