In searching for valve guides for my ECZ-G heads, I find that there are two different lengths of them, with no explanation(that I can find)as to where and when the two sizes are used. One size is 2 3/8" long and the other is 2 1/2" long. All other measurements are the same. Can anyone tell me where the different lengths are used? What is their specific application? Both sizes are listed for the 1957 heads. Also, there are smooth bore guides and spiral bore guides. Which is the best to use? Does one have an advantage over the other? Would the spirals interfere with the normal valve rotation? So many questions - I am confused! Any help will be appreciated. Thanks.