I installed a 1967 Mustang drum brake dual master cylinder in my 1955 Thunderbird yesterday.  It bolts up perfectly and the 1955 rod fits into the newer cylinder.  Be sure to use the 1955 button on the end and you should have no problems with the rod length. 

The brake rod length for the '55 will operate the dual master cylinder. The rod length
should be checked, as the rod may not be long enough to activate the secondary cylinder.
There was a lengthy article a few years ago with details of extensive testing of this issue.
Ernie

I haven't personally tried it, but everything I've read says that 68-69 Mustang will work, just 2 bolt vs 4 bolt installation and the associated plumbing.

Don

I wish to upgrade the brake master cylinder in my 1955 Thunderbird to dual bowls.    I did this to a 1957 Ford custom a few years ago and I used a 1969 Mustang drum brake master cylinder.  I did not have any problems with the brake rod length.  The stock 57 custom rod was the correct length and worked fine.  Has another tried this on a 55 TBird?