I have run spring blockers on the street in my car for close to 100,000 miles now , one per side rides fine and gets rid of the ass sag these cars are known for.
to the person asking what they are- all the gm cars built by holden for sale in the us such as gto/g8 got red polyurethane spring blockers to keep the cars from pitching around on the transport boats. most of these got thrown away at the dealer level allthough a few guys found out how well they work installed in the rear springs of fwd cars to limit the amount of weight transferred off of the front wheels upon launch , 2 per side works decent at the track.
at the track I prefer the cable mod since it allows no weight transfer off the front wheels but you dont ever want to use it on the street and you could fail tech if you caught a sharp inspector due to no rear suspension travel
spring blocker installed:
cable mod installed on a car I parted out due to rust , took pic for someone wanting a visual of cable mod
I use the old school style spring clamps/raisers. they have 2 bolts and like U saped brackets that fit the spring dia. You can get at any parts store cheap. I jack the back end up off the ground, install and adjust the nut so I get them to there max seperation point then lower the car down. repeat on other side. I use 1 per side too. Although it looks like I raised the rear alot once its settled its about 1 to 2 inces higher and minimizes the rear travel substansially. Look at this pic launching and you can see the rear isnt that low. 2.19 60ft here.