I have finished mocking up the brackets for installing the 240SX calipers. The caliper is from a 1998 240SX with a 38mm piston. This conversion gives a 67-33 bias. It seemed to be well balanced for the GT though it may have too much rear balance for a roadster. I may find someone who wants to experiment and try the conversion on a roadster.
I do like the hydraulic and park brake cable routing better with this caliper. I did take it out for a spirited test drive. The streets were still wet from the previous day's rain and there were dry patches. There were no surprises even with hard braking in curves. One thing I really liked is the nose dive is greatly reduced. In hard braking the nose dips slightly and the entire car feels like it just wants to squat down. The temps at all 4 wheels was very close. I found a hydraulic leak so I will have to fix that this weekend. I will take it out for a more extensive drive after that is fixed.
Feb 6, 2018 update
OK I'm happy with my rear disc brake conversion. Bob Harper in Australia was kind enough to correct the drawing for the Sentra/Sunny caliper bracket. The drawing has been checked and to the best of my knowledge it is correct. The dimensions have been changed to make it easier to layout on a piece of steel or aluminum with easy to locate reference points. I'm not going to develop this conversion any farther.
If anyone wants the drawing for the caliper bracket send me a PM with your e-mail address and I'll send the drawing along with part numbers for the caliper and rotors that I have used. As far as the additional bracket for the 240SX is concerned, it is easy to make at home out if 1/4" flat steel stock so I'm not going to commission a CAD drawing at this time. If someone wants to make a CAD drawing of the additional 240SX bracket feel free to do so and share. Also feel welcome to use the finished drawing as a basis for any DIY rear disc brake conversion you care to undertake. All of the basics are there.
Update Feb 27, 2018
FYI. Using a spare prototype bracket I did a 34mm Sentra install on a 77 RBB roadster. There are a couple of challenges. All but one can be resolved easily. The biggest issue is the boot floor for that car is different from the CBB GT. There is interference between the park brake linkage and the boot floor. This weekend we are going to experiment with a different park brake lever on the caliper. Failing that the 34mm 240SX caliper will be a better choice since the park brake lever on the caliper is located close to the axle housing and not out the back.
We did take the car for a spirited test drive on a rainy day and we feel the 34mm caliper is well suited for the RBB roadster. We also took out the CBB GT and feel the 38mm 240Sx caliper is a good match for that car. The early 240sx, target date 1992, has a 34mm bore and the later 240SX, target date 1998, has a 38mm bore. Both will use the same add on caliper bracket since the exterior and mounting dimensions are exactly the same.