I've got a 66 Jetstar 88 and I'm considering the same upgrade. My plan is to use 67 88 brake parts. In your case you'd use 67 Cutlass parts. Booster & master cylinder should be a bolt on. I don't think the 67 MC's will fit the 66 booster. Your 66 brake pedal should be OK. Wheel cylinder should be the same, but get a parts store to check it.
Brake lines are where the problems are. On the 66's the line from the master cylinder went to a four way connector block. (MC, 2 front lines & 1 rear) On 67's the block is a 3 way for the front only. (front MC & 2 front lines) This 3 way block is not reproduced that I know of. I plan to get one from a salvage yard. Then order 67 brake lines from Inline Tube (
www.inlinetube.com) About $100 if I remember right.
Some people believe the single reservoir systems are safe.
If you are considering front disc brakes this might be the time to do it.
This is the major safety feature on your car and your son will be driving it. Please research whatever plan you decide on. I'm just passing along my plan for my car and how you might adapt it to your car.
Don