I agree that this may be a marketable product, but the key to low cost is volume. I don't think you're going to sell 1,000 heads, which means the R&D, molds, tooling, etc. are amortized over a small run.
The usual problem with head design:
A. suit all existing parts (highest sale volume, lowest cost to use)
B. as many improvements as possible (race only = much lower volume, much higher cost to use)
In my opinion, none of the commercial manifolds are good enough to warrant duplicating the existing intake port layout. An L6 is (relatively speaking) very easy to design since it's been done 100 times - an excellent example is the stock Sprint manifold. In fact, an exact duplicate will probably work very well and shorten development. It's got the right cylinder spacing (4.40"), and the Sprint intake ports are pretty good size.
I suggest dispensing with the stock pistons as a feature - no valve reliefs, etc.
The intake valve should be very large, and centered as much as possible in the bore, with the exhaust taking whatever is left (that's almost a quote from John Kaase) - this is where current engine development is pointed. How close the exhaust can be is partially a temperature question - how close can the seat inserts be and stay in place.
Making the casting taller than stock raises all the ports, and allows a big short-side radius, but will need taller valves and pushrods.
I suggest using BBC rockers, and install the 7/16" studs with generous clearance for the pushrod.
