Which conversion kit to buy?

He does. You can buy the brake kit here and add everything but the spindles and UCA's 10.95" Front Disc Brake Kit (Stage 1)

He sells the spindles and UCA's as well Mopar Reproduction Disc Brake Knuckles for A/B/E Bodies and Mopar Upper Control Arms. Although you could also just buy is ball joint adaptors and keep the small ball joint UCA's you have.

So, its not a full kit per say that you can click one link and order it all, but he has everything you need. I'd just call him and tell him what you're doing, Cass @DoctorDiff knows what you need.