RE: George Miller F-4 Build - also, welcome George!
The Miller Phantom is a great flying jet!! I've built 3 of them (2 with a single Dynamax/OS.91s and 1 with twin Dynamax/OS.77s
I made my own bifurcated thrust tubes (for the twins) out of 1/64th ply rolled and glued in the jig shown below.
BTW - I looked all over and can't find any of my plans. They were all of the later version of the kit.