Convertible frames are dimensionally interchangeable with Coupe frames with the following exceptions:
1. Convertible frames are boxed under the passenger compartment and use a different tranny crossmember, fuel line set, and front to rear brake line.
2. The two bodymounts at the mid-door level on a convertible frame have bottom bushings and thru-bolts while the factory Coupe configuration is just a floating bushing on the top. This isn't so much a frame difference but an assembly line difference; coupe bodies have nutplates in the bodies in this position and you can add additional bottom bodymounts and bolts here and get some extra stiffness.
3. Convertible frames use two additional body bushings up front under the cowl.
Elky/Wagon/4 door frames are also boxed. However 68-72 versions have a longer wheelbase than two doors-- the stretch is done in the center. Elky frames also have a longer rear section. The rear section is easy to chop and shorten but taking the extra wheelbase out of the center is much trickier not to warp things.
Best bet is to either find a complete convertible frame (often very expensive) or just to box your existing (or another) coupe frame.
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1969 Chevelle
Old setup: Procharged/intercooled/EFI 353 SBC, TKO, ATS/SPC/Global West suspension, C6 brakes & hydroboost.
In progress: LS2, 3.0 Whipple, T56 Magnum, torque arm & watts link, Wilwood Aero6/4 brakes, Mk60 ABS, Vaporworx, floater 9" rear, etc.
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