Fan shroud size vs fan blade

[John]

So I am very pleased with the cooling now I get with the new Dewitt's Aluminum 2 core with 1-1/4" tubes. However, I need to address one last issue. My fan shroud and fan blade sizing isn't correct. The shroud measures 21" wide (best I can tell - not easy measuring with fan in place). And the blades appear to be 18". Too much gap between shroud and fan and not efficient pulling. What I assume is the shroud is a big block shroud and the blade possibly for small block. What is the largest stock 772 (7 blade) GM made? 19" 19.5"?