Well I finally got around to installing the longer rear spring bolts (10 inch vs 6 inch stock bolt, car is on left side of pic) and dropped nut till it was one inch from end of bolt, and at the fender ride height was 28.5 inches, I would like another.75 inch drop all around, I know the spring will "settle in" after awhile but I have new springs up front also, and want to know best way to drop height at front.

 

A lot of Corvette owners, including those "restoring" historic race cars, make the mistake of changing ride height without regard to suspension geometry and its effect on mechanical grip. If you're looking for optimum handling, you should trim the front and rear ride heights according to the Chevy Power manual (C3) Corvette Chassis section recommendations, as follows:

Measured on level ground and with vehicle full of fuel and driver's weight on-board, at the front you should have a measurement of 1" - 1 1/4" from the height of the bottom surface of spindle support on the lower ball joint to the height of the center line of the lower control arm front pivot bolt. This is refered to as the "Z" height. Your ride height is probably not this low unless it's already been set up to these specs or, more likely (for old springs), the springs have sagged and are due for replacement.

There are two preferred ways to get there, neither being easy, but I highly recommend cutting coils a little at a time with a carbide blade to the dangerous and imprecise method of "bulldozing" the spring. (Don't cut the pinched end!) This is a trial and error process, so go in small increments as you can't undo a mistake if you take off too much at one time. Also, you might as well figure this is a good time to replace the ball joints, but not before doing all the fitting first. In any event, use proper spring compression tools to avoid injury.

At the rear, again with the tank full and driver's weight on-board, you are looking for a measurement of 1 1/4" between the heights of the centerlines of the rear camber strut rod ends. Called the "D" height, this one's easy with the long spring bolts. Don't be concerned with long term sag here until it occurs. Better to have the geometry correct, and adjust as may be necessary down the road.

NOTE: Be sure to settle the suspension back down by either driving around the block or bouncing it a fair amount on free-floating turn plates before taking these measurements after each adjustment. If these ride heights present a problem with ground clearance, you'll need to go back to the standard factory settings for your specific vehicle combination.

One mod that every Vette owner should consider doing is to replace the stock rear camber strut rods and the unreliable concentric adjuster with a quality adjustable rod set up such as VBP's poly-adjustable strut rods. You can always save the originals and put them back on. Also, you ought to consider a mono spring. Loose about 40lbs and get improved ride quality.

All that said, you should utilize the shortest spring bolt which yields the correct adjustment, which should most likely be the 8" in your case.

Hope that helps

 

I lowered my 70 with new front springs by cutting off 1-1/4 coils that gave a 2" drop. 8" rear bolts were adjusted to match the front.
I Autocross and the car handles fine.