Sure it can be done! I've seen El Camino convertibles, etc.
The biggest challenge is getting enough bracing to keep the body rigid. You have to add extra stanchions, a rib or two to keep the sides from wavering, etc. Of course, welding the rear doors shut will certainly help. I would try welding the doors or adding sheet metal and getting bigger doors from a similar model, although they stopped making 2 door CVs in the
80's.
I'm sure there are racing shops in you area that build cars. They would have the know how as to how to make a car like this happen. Start picking brains if no one here gives you a good answer.
There used to be someone in the Dodge group calling himself Refinish King, he may have an idea or two also.
I cut part of the roof off my 91 Bonneville June 2008. Its cut from about four inches inward from the rain gutter and up to the visors. The rear window was removed as well. I still have the center pillar and window frames intact. My seat belt are anchored to the top of the window frames on the front doors - thus the reason for leaving them on.
I had a top show make up a cloth top that snaps on like the boat covers you see at marina's. I can drive it the rain & snow. Made five cross bows out of conduit to support the canvas. There is a plastic window sewn in the back.
The car has gotten alot of horn honks, waves and shouts over the last year.
I'd love to do another car as the Bonneville is nearing the end of its life. I've noticed no body flex or problems since cutting the roof off. Of course the d-ring (door jamb area) hasn't really been compromised, I don't think.. so it seems pretty solid yet.
The neat thing about doing this to a large car is you have a car that has lots of room and a decent trunk size. All these new convertibles out there are essentially two-seater cars. I can fit 5 adults (not kids) comfortablly in the Bonneville.
Considerations for the next chop are 97-05 Bonneville's, LeSabre's or Park Avenue's because these models have the seat belts attached within the front seats frame. The entire top, center pillar and window frames of these models can then be removed. A 2000 & up Monte Carlo is also a consideration but the seat belt on the center pillar comes into play again.
I've also considered the Grand Marquis, Crown Victoria or Town car, but some of the center pillar would need to be sticking up because of the seat belt attachment.
I'm leary about buying a real old car. I wish I had done this 30 years ago when the Big Three build those big 4 door hardtops that had no window frames on the doors or center pillars connected to the roof.
There are several Video clips on YouTube from people that have cut off tops of cars - just search around and you'll find them.
I just _can't_ lurk for a while on a newsgroup! I just read a post and dive right in...
Pretty much since day 1 closed cars have gained a considerable amount of body rigidity and strength from the roof. With new cars needing to be light and crash-proof and all the rest this trend has only intensified.
So when stand there with your sawz-all eying the roof you're contemplating cutting of a pretty major structural member of the car; if you want to close the doors when you're done you'll have to replace that strength before you challenge the car with anything even as mild as _opening_ the doors.
Do Crown Vic's have frames? If so, then your life is eased somewhat, in that you 'just' need to beef up the frame and (as already mentioned) brace the door posts so they can't wiggle out of place. If the frame is channel, boxing it from the cowl to the rear window will probably suffice.
As soon as the roof comes off your door posts will be floppy; you'll need to reinforce them _and_ the back of the cockpit. Cars like the '36 Ford Phaeton and the '40 convertible did this with great big forged members that braced the door posts to the frame; this works if there's a frame member that can carry the load but then your road-noise suppression gets in a big fight with your body rigidity.
Everything is complicated by the fact that you want strength right where you can't have material. The best way to do it from a purely reinforcement point of view would be to run a member straight across the car, but your passengers might object. This leaves you with needing to make big "U" shaped members that don't weigh much, don't get in the way, and won't let their tips move appreciably even when you're exerting considerable force on them.
Your job on the middle door posts will be complicated by the fact that you need to lend strength not only against in and out motion, but against fore and aft motion as well.
I'd seriously consider a member that went behind or under the rear of the front seat that tied the two door posts together -- figure that it has to stand up to someone slamming a front door at the same time that a ten year old is swinging on an open rear door.
And finally, keep in mind that you can't learn stuff unless you screw stuff up, and you can't screw up a project unless you start it.
However your looking at MAJOR problems. First you need to stiffen the frame under the car, then add in more body mounts to secure the center section of the body. Now you need to add in structural parts to support the door posts and those HEAVY doors. Then you need to come up with places to mount the required shoulder harnesses. Now once you get done with all of that you will need to take it to the DMV or whoever does your states/provinces vehicle inspections to get all of what you have done approved. Then you will be able to title/register and insure this vehicle.
Hopefully the guy with the Crown Victoria lives in a state that doesn't require state inspections or having to bring your car in someplace for license plate tags.
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