I have been doing a lot of research on putting a 700R4 in my '72. There are many ways to do it according to the trans shops and speed shops in the area. It can be wired with the switch so you can shut it off when you put your foot in it. It can be wired to the brake switch so it shuts off if you hit the brakes and it can be set up to always lock up in high gear. But if you slow down, it will lug at slow speeds but the trans will most likely shift down to 3rd or 2nd and unlock it automatically. And if you put your foot in it, it will downshift to 3rd and unlock. Remember, when they put these in cars as standard equipment, they had no switch on the dash so getting it wired to work automatically is the least bother unless you are racing it.
Also, get an '87 to '92 trans. Prior to that, the shops said they had a lot of problems and after they they were electronic.
I hope this helps. If you want my research, I have it ina word document that is too long to post here. Send me your E mail and I'll forward it to you. Keep in touch in case we have questions along the way. Thanks, Jim