'67 Barracuda Rear Leaf Spring Lowering Kit

AFCO makes them. Just get a set that has provisions to take care of the locating pins so the blocks don't slide out. They should fit over the pin on the spring side and have a pin to fit into that axle on the top.

As for the "best" way, that would be to have springs with the right arch. But if the blocks are 1" or smaller it's not a problem.