Paste, IMO, is the better option. Pads are ok if you're hot swapping coolers, for less mess. But when you're going to leave the cooler on, why mess with pads.
Also on the pads, I think you want to measure the CPU not the block, as the block will overhang the CPU.
Make sure the fans you are mounting secondary match the already installed fans. You don't want them fighting each other. Or causing a restriction.