383 Chev,

A couple of things. The engineers at the factory spent a whole lot more time and money trying to determine how long or short the plug should be than you or I could possibly do. It is best to use somewhat close to what the factory used in the particular application.
As far as "I don't think so" for the Autolite plugs, I could tell you a story about that but it would take too long. If Autolite plugs are available to me I will certainly use them. They were the only thing that I found that would hold up on a brand new 1969 Camaro 396/375 SS for more than 300 miles without missing. That car was death on spark plugs.
As far as the Holly carb, I have use a lot of them. Thay can be a royal pain. If you can find someone that knows how to set up a Q-jet, they are a very dependable carb and will perform very well.

Ed