The twincam head for the [email protected]* is only manufactured by OS Giken and *not* Nissan. I think you both are thinking of the S20 engine that came in the Fairlady 432's. The twincam head from OS Giken runs about $30,000.
Why not just go for RB26DETT? That has a twin cam, a twin turbo set-up and only goes for twin 5,000 dollar bills. That includes full engine (running), wiring harness, and ecm. In Japan they are only rated at 280 Hp, but that can be changed easily, by calling HKS. Have you seen their Skyline?
JG Engine Dynamics had one of the O.S. Gikken heads and did some dyno work on it. As a bolt-on, the gain was not really worth it. They could build you a complete SOHC engine and still save $20K over the cost of the head alone. I've seen them in Japan, and they're really trick, but the prices everybody's quoting was from 1985, when the yen was 268. Now it's 121 on a good day, DOUBLE the price in the Nissan book and then try to FIND one. Buy the RB, it's the next generation of the L-block anyway.