The parking brake on my 69 T50 is cable operated. With a really long cable and some custom made brackets, it could be adapted to bolt to the front of the diff... Never done it or seen it done, just thinking.
BTW, I love the TH400, had one go into reverse at about 60 mph once. NO problem, made a lot of noise with the wheels hopping and squealing, the diff eventually got noisy, the u-joints gave up pretty quick, but the the trans never complained in many miles since.
