Billet?
If you're going to build something like this, there are two possibilities that seem to be most appealing to me. I'd rather have a cross flow head. Not sure about valvetrain layout - a pushrod design could keep a lot of stock-fit parts, be easier to install, and save money, a DOHC 4 valve design would be cool for all out power but way expensive. My vote for the pushrod design would be to use shaft-mounted rocker arms, but in a size that lets you use common rocker arms from some V8.
It would be convenient if whatever was on the driver's side could use a stock manifold, but this may be a bit of a compromise when you no longer have to stick two manifolds on one side. Maybe you could build it so that it used BMW M50 or S50 manifolds on both sides? They're a reasonably common slant six design, although I haven't checked the bore spacing to make sure they'd fit well.
One other thing - a crossflow oveheard cam head absolutely must have provisions for either a distributor drive or a camshaft position sensor for EFI and distributorless ignition. The stock distributor won't be of much use with a crossflow head. (I'm not entirely sure what I'd do with a pushrod / crossflow design. Probably stick a cam sensor in the fuel pump drive slot and replace the distributor with a freeze plug.)
Not sure I'll have the money for one any time soon, but it's a cool idea.
