NJRR - I'll start from front to back on differences
- front sheetmetal is interchangeable from any SW, 4dr, 2dr Plymouth B body line....that's the easy part
- Firewall, upper cowl, inner and outer door hinges are the same across coronet and belvederes....66-70
- Front framerails the same 66-70
- Front frame torque box - not always there...I have a 68 convertible that doesn't have them....regardless it's available as an aftermarket part
- Rockers.....convertible specific. Below the outer rocker is a 1/4" formed steel plate that runs the length of the rocker....it would be hard to put one into a hardtop
- Windshield to upper cowl plate stamping (where the windshield attaches...the leaded seam), windshield frame, windshield chrome surround - 68-70 convertible specific
- Windshield - I've heard that the 67 sedan and convertible windshield is interchangeable but haven't confirmed...otherwise 68-70 convertible are the same across the coronet and belvedere lines
- Doors - glass and vent frame post is different. Shell looks to be interchangeable but needs the brass wedge at the back where the door latch meets the door post
- Door latch post - convertible specific
- Rear windows - convertible specific to the 68-70 coronet and belvedere lines
- Rear interior steel panel that the window regulators mount to is convertible specific to 68-70 coronet and belvedere lines
- Dutchman - convertible specific to 68-70 coronet and belvederes
- Quarter Panels - the top portion is convertible specific and are the same across the coronet and belvederes just around the convertible top area....you would have to splice a quarter panel at the top ridge line
- Wheel tubs - they have a cap that is convertible specific but the lower portion is the same. Aftermarket parts could be modified to accept the cap. I know that 68-70 coronet and belvedere parts would interchange....I don't know about the 66-67
- Sheet metal in the convertible boot well area may be interchangeable from 66-70...definitely 68-70 coronet and belvedere
- Rear torque boxes - common across all convertible B bodies 66-70 and are available aftermarket
- Spare tire mount is unique to convertible
- Trunk lid hinges, hinge brackets, and torsion springs are convertible specific
- all the rear window tracks and hardware are convertible specific
- Rear frame rails, trunk floor, gas tank trunk lid, tail lights.....common across the 2 dr and 4 dr belvederes
- Top frame - 68-70 coronet and belvedere
The short version....find a 68-70 B body convertible to start with and hang the rear quarters and 70 sheet metal to it. You will be light years ahead.