When you say the brakes are marginal do you mean that it takes both feet on the pedal and hour butt in the air to stop the car?
That kit lists a 1-1/6" master cylinder. That's a big master for a stock style caliper with a manual pedal. Did your pedal have two holes for the master cylinder push rod? If so, check the position and make sure you've got it set correctly.

We are brothers and sisters doing time on the planet for better or worse. I'll take the better, if you don't mind.
- Stu Harmon