Hi Fred,
I've been searching for this part for some time and have had no luck. If I come across any, I'll let you know, or if you do, please let me know.
As far as I know, per a previous posting, the Tee is 5/16-24 Inverted Flare x 3/8-18 Inverted Flare x 1/4 Male Pipe Thread into the Manifold; I've posted a photo of one that is not correct but close in appearance. Making it from two Fittings is easy but not Concourse Correct.
My next step is to contact one of the manufacturers who make this type of Tee to find out if they will mill one and if so, how much will it cost. There's also a shop in Oklahoma that restores Mustangs to Daily Driver Standards. They convert all the sixes they buy to eights but attempts to get them to respond to my parts request netted me one response; I answered and haven't heard from them since.
Ray