Re: Aluminum exhaust manifold
The cast iron manifolds are prone to cracking (from heat) and letting water in the #4. The aluminum manifold didnt have this problem but the problem you are going to face trying to put the aluminum manifold on is the angle of the exhaust exit. Your 1970's version exits straight down, while the 1980's era cast iron and aluminum manifolds exit at 45 degrees. You will either have to use a later transom exhaust tube (which means removing the engine) or fabricate an adapter to mate the later manifold to the early style exhaust pipe. There isnt anything commercially available to do that so you will have to come up with it on your own if you dont want to remove the engine.
There is quite a weight difference but other than lifting it to install it, like Don said you arent going to notice any difference in real life. Unless you want to deal with the exhaust studs, different hoses, and adapting the late manifold to the early style exhaust pipe you might be better off looking for a used cast manifold. I see them on ebay and craigslist fairly often, they arent too hard to find.
