Slantvaliant provided the info I was looking for (thanks!)
It involves making up new brake lines to plumb into the old
junction box.
Today at the junkyard I found a rust free 69 Valiant (9" drum car)
I took all of the brakes lines and the 69 junction box in one
piece.
Is there any reason I couldn't use this on the early car (66 BCuda)
with a new drum brake m.c. to upgrade my system?
Does the later juntion box mount onto the inner left frame well in
the same place?
This would save me from buying flaring tools, coupling pieces etc.
The old lines look very good (from the rust free jkyd car)

This should be pretty much be a bolt on. The junction box will not be in the same place. Some minor hand bending may be required.

You might be able to use the newer lines and distribution block, but be careful if you have to bend the lines. Use a tubing bender, and be careful to get the bends right the first time so the line doesn't work-harden and crack.
I haven't checked locally, but some of the parts stores lend or rent special tools, and might have the double flaring tools and bender.

I used brake lines from a '73 A-body in my '65. The only bending that I needed to do was to adjust the lines where they meet the inner fenders and to match the contour of the firewall. The was more along "adjustement" bending. While a tube bender is needed to make sharp curves most gentle curves can be done by hand.

Good to know!