I have TTI's on my 65, doesn't look to me like the master cylinder is any closer to the headers than it was to the 340 manifolds I used to run. I did fabricate new brake lines from the m/c down to the combination valve, which I had to move to the rear to get it out of the way of the headers.
Curious if you have any pictures from below. My stock 273 manifolds are RIGHT on the proportioning valve. Haven’t had any issues in the short time I’ve had it on the road, but I do know I need to address it.