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How to Retrofit a late Convertible Hardtop
Compliments of corvetteronw @ www.corvetteforum.com
RETROFITTING A 1989 THRU 1996 CORVETTE CONVERTIBLE HARDTOP TO FIT THE 1986 THRU 1988 MODEL:
In March of 1989 production was started by the ASC Corporation of Southfield, Michigan on this beautiful 64 pound top. It was designed to be bolted on to the body after the folding soft top had been stored underneath the rear hatch lid.
The hardtop will bolt to the earlier C4 models but three things need to be changed:
1. With the help of a donor 1989 thru 1996 Corvette (either coupe or convertible) you will need to swap in the front latching system and the upper plastic trim piece that runs between them.
2. You will need a donor 1989 thru 1996 convertible to for its rear deck lid. You can remove the hardware from the later deck lid and swap things over but I find it easier to just swap in the entire piece.
3. The GM authorized hardtop has a built-in aquare roll bar that attaches behind the doors with the help of a dog-leg bracket that ties the frame of the car to the hardtop's roll bar. There is an aftermarket top that is made by Smoothline Industries but it does not incorporate the roll bar or its attachment points.
Since there is no place to attatch this dogleg brace to the earlier C4 body you will have to fabricate one. First remove the bolt that secures the shoulder belt to the stancion adjacent to your shoulder when you are seated. With the hardtop in place you will make a cardboard template in an L shape with 2 spots to be drilled inr the vertical portion of the L to be secured to the square roll bar. The horizontal portion of the L bracket will be drilled with a hole that will allow the shoulder belt to be reattached and secure the new bracket. The driver and passenger L brackets will then secure the frame to the hardtop and give your convertible a more solid feel.
Take this templates to your welder and have them fabricated out of heavy steel plate.
Installation can be done by 1 person but it is easier with 2. Lower the convertible top and the rear deck lid. Flip the sunvisors down. Set the hardtop onto the car and loosely bolt in the front upper windshield bolts, the rear 2 bolts that take the place of the soft top pins, and The brackets behind the doors. If your hardtop did not come with an installation tool then it is a good idea to purchase a ratcheting wrench (flat 8mm for the rear and the appropriate size Torx bit for the front). Once everything is lined up then snug the bolts down and enjoy the new quiet and rigidity of your cool weather cruiser.
Just be aware that you will lose easy access to the storage space behind the folded convertible top. Pretty much leaves enough luggage space for a skimpy nightie and a toothbrush for your partner. 
Here are the stock side brackets:
The fabricated brackets installed:
Had to redrill as I did not measure correctly:
Lookin good!
