more than just blocks
Replacing the blocks with shorter ones is one piece of the puzzle. There are other factors to consider if you want to do a quality lowering job.
U-bolts have already been mostly discussed. Yes you can get away with reusing your stock u-bolts if they are pristine, but they will be too long, thus prone to catching stuff and becoming damaged. Correctly sized replacements are best.
Driveline (pinion) angle changes with shorter blocks. This misalignment can result in driveshaft vibration, which is an indication of accelerated wear. Any quality lift kit comes with proper sized (angled) shims to correct the pinion angle. A quality lowering job needs to do the same.
Brake proportioning. Our truck have a brake proportioning sensor that biases brake proportioning to the rear under load. This sensor is based on pinion angle, no scales involved. It can be adjusted to be normally biased at lower ride height.
Headlight adjustment. Our headlights are pretty much crap as is, pointing them higher doesn't help and only aggrevates oncoming motorists. They should be adjusted too.
Most of these considerations are nominal to no cost, but valuable in the overall quality of the job.
Cheers,
Neil