Don--------yes sodium filled ex valves are quite common---------


If you want to go more modern, look at the GM RO7 heads for nascar---------


And , I didn't mean to get the 92-97 for emissions so much as the computer and o2, and other sensors gave good control of the engine--


The impala engines were iron heads where as Camaro.f bird and vet were alum -------
you can use the latest ecm E38 which gives more control over about everything-----


My son Corey is doing dyno tuning dailey with the new Camaros, etc where they are making around 800 rwhp , idles around 800-850 and gets excellent mpg IF you keep you foot out of it-----


We have built dozens of the LT series engines over the last several years, but they are getting hard to get an adequate number of core blocks, but, there are wrecked complete cars that can be gotten reasonably for complete core package including wiring harness, fuel systems and engine driven accessories-----you can even do ride control with the right shocks from these vehicles------------