Some things I have been thinking about, as well as those I mentioned earlier, is are you sure that your Tilton Bar is properly set up. I remember when I first installed mine it wasn't quite correct and had to be adjusted.
Changing the diameter of one of the master cylinders shouldn't be needed and I don't think I have heard of anybody with a 289 doing this as the Tilton Bar should compensate for brake bias.
What brake pads are you using? Some pads such as Green/Yellow/Red Stuff require the pads/discs to be hot before they operate to their maximum efficiency. I use Ferodo DS 2500 pads which have more bite from cold and are not prone to brake fade when hot compared to standard road pads.
Apart from fitting a servo I cannot think of any other way to resolving your problem, but a servo will only reduce the amount of effort required to stop the car not how quickly it will stop.
I have attached the Brake Test report for my car at it's SVA which you might want to compare with any results you have. The Tilton bar was set to maximum for the front brakes and minimum for rear using Princess 4 Pots with ventilated discs at the front and Jag IRS rear axle.
