1 7/8 Headman Small block header question

I have a set of these headers and was wondering if you own a set did you try muffs or extensions on these headers. If you did, did you pick up any performance? They seem like they have a short collector. I have them on a eddy headed 408. Thanks

i've found on "most" stroker smallblocks, a total collector length of 16-18" seems to work best. measured from where the tubes enter the collectors.