I have the origional stock mono leaf suspention on my 69 and It is starting to sag alittle bit in the rear. THe car has dropped an inch or 2 by my best guesstament. My question is will replacing the old tired mono with a new multi leaf hurt my 60 ft times? or will it help? I run a consistant 1.8 60ft on slicks with no other traction device.

Depends on the build of the spring. Look toward the Hotchkis springs for replacment.