Coil overs will make the situation worse. So now point is you had a diesel chassy with gasoline engine in it. Still the suspension is jumpy desighned for diesel loads.
Now the question arises are the shocks refilled ones or the gas filled ? If these are oil filled then you will be inable to enjoy the torque of jz on sharp curves because of soft suspension on front.
Fo coil overs go for 4 wheel one not just front or rear ones. The. You will enjoy the ride and one thing more dont forget to check the rear end bushings and differential bushes.
Plus as the groir is desighned to lean on front side do the opposite setting on coil overa lower on the back end then the front i had done to my groir the same back in 2010. And enjoyed the handling a lot.
And one last and important thing upgrade the brake rotors on all wheels find the cross drilled large rotors easily available woth Rahat shah at cheap rates as no one install them. And also upgrade the front brake booster and cylinder assy. Stock groir has a 1/2 inch master cylinder try to find the 1 inch and you will feel the brake on a touch witout skidding your front tyres.