Depends on how big you make the tops and how much of the 'middle' of the canopy is left. My old car had T's - homemade by the PO (and he didn't save what he cut out *very frustrated*) and I had no issues with the structure of the canopy. The center spine was about 4 or 5 inches if I remember right. There was some noticable flexing, though, going down the road but it didn't seem to bother anything... and I drive that car hard.
That said, there isn't any easy way to do it with an already built car. I know Dave and previously Mike McBride put "S" fiberglass - a much tougher, denser weave - into the pillars when they made the canopy halves for the open top canopies. I can't think of any easy way to retrofit that short of drilling a large hole at the top of the channel, stuffing S glass (or rolled into a tube) down the channel as far as you can get it then pouring the epoxy into the channel enough to fill it... and hope it saturates the fiberglass enough.
A simpler, though a little more space invasive way is to put a simple metal support from the middle of the frame at the top down to the dashboard. My Manx had one and the rearview was mounted to it. It wasn't to obtrusive, just looked a little "kit-car-ish" even though it was that way from the factory.
Dave doesn't have T-tops, just the targa roof. I don't think he will offer one, either, due to the complexity of making the actual T inserts, even though he has an open offer of mine to use as a pattern.
Here's the dune buggy support:
My first car in it's raw state. Yeah, it was a "T" top *humpf*... center spine was as wide as the handle opening:
And later in it's life with a metal ring support all the way around the opening:
And the Rose car with a factory T:
So.... it's not so much about cutting the roof, but making it watertight again with the inserts. Probably one of the neatest solutions I saw was in a newsletter 'back in the day' where one owner took a set of his and hers sunroofs from a van and installed those. Those used to be really common... sometimes they come up on eBay from time to time.
No close up of one open so I can see how they ran the weatherstripping*thumbs down*, Oh well.
I was just thinking of a valley and peak thing. The peak would have the weatherstripping.
The weatherstripping on the inserts would fit into the valley on the canopy and vice versa.
Have the valley on the canopy deep enough to catch water and somehow run it out of the car.
Or you could install a glass roof. One big sheet of tinted glass. The underlying structure would still be there, just out of view. Once completed the roof outside would just look like it had black paint job on it. But in side it would brighten up the cockpit alot.
Let me dig through my Carlisle photos. I may have caught Dave's weatherstrip in one. It isn't much, a simple seal front and back that the targa simply sits on. There are no rain valleys to speak of, even with the targa piece on.
I have 1 of Daves targa tops. which seal are you talking about? the one around the lift out targa or the whole canopy? the canopy just has a typical d shape strip and I am waiting on him to send me the rest so I really cant help with the targa. I could take some up close pics of the top if you want.
Hmmm. I've always been thinking of that for the Sebring, but worried about lack of support for windshield. And, of course, part of the great thing about these cars is the top - and chopping up the visual flow to add a sun roof or one of these, may really mess it up.