when you said a 38 might be a little big I was looking at the bike and the down tube looked too close to the cylinder head so I got my tape out. As near as I can tell, the down tube is 4 1/2" from the carb mounting surface. The stock carb is 4 1/4' long that leaves 1/4" for an intake manifold. obviously a no go. This is 79 Special 650, is there something different about this frame? I can't see how any manifold/carb combination is going to fit without replacing the single down tube with a BSA type dual tube setup. Man, it's always something. Fortunetily I haven't purchased anything yet and won't until I get this figured out.