Tom have you tried adapting a Pro Chamber? I don't think anybody makes these for classics, but there's a lot of Mustang guys that swear by them. Especially the SN95 mustangs suffer horribly from drone issues.
It produces a deeper tone more like an H pipe, but because it's effectively a helmholtz chamber, it turns whatever headers or manifolds you have in front of it into something more akin to a 180 degree header.
