I think you're thinking of the 516 heads, which are closed-chamber but have smaller exhaust valves and restrictive ports compared the 915s. I can't say I've seen/heard of a 1.60" exhaust 915, but I'm not into big blocks like I once was. If there is, though, I'd assume it's the same situation as LA 915 J heads: The only difference between 340 and 360 casting is the machining. The castings are identical.
No sir. There are two distinctly different 915 big block castings. Only one is the 67HP casting and that's the one with the 1.74 exhaust valves and better ports. I've had both in my hands before as well as the 516s. I'm very familiar with them.
