Anybody ever cut the T-top roof off a donor car and weld it on to a hardtop car?  Any thoughts and opinions on this matter are welcome.  

What year and model? If it is a second generation Firebird/TA, use the donor car as a template and cut the openings and use all the t-top hardware. I saw a car once that had a transplanted roof, it wasn't done very professionally, leaked like a sieve and had lots of bondo at the joints, and probably wasn't very safe to boot.
Steve  

A 79 TA would be receiving the T-tops.  I talked to Clayton Custom Cars out here in NC and they said they would cut it off at the seams just below the roof on the b-pillar and weld the entire T-top roof on at the seams.  I think they mentioned something about doing a little overlap as well.  Is there any structural difference between the 2 roofs?  

I've been researching the t-top swap myself, since I always wanted some. I just bought a donor t-top roof and panels from another 79 for one of my 79's.

Your custom car shop is correct about replacing the whole roof for your 79.

I would brace the inside of the car before removing your roof to maintain your dimensions. This means removing the interior to be able to tack weld some metal tube bracing in temporarily.

You want to use the factory Fisher t-top roof from a donor car, a 78 to 81. There are factory seams at the rear where the old roof would be removed and where the new roof would be welded back in the factory position.

The front posts require more study. When you get a t-top roof, get the front pillars cut below the bottom of the posts, so you have plenty of metal to work with. There are several layers of metal layered in the pillars. So far, it looks to me like the best way to put them in is to remove the lower section of the outer skin on the pillars, which makes it easier to graft the inner structure to your original inner structure for a strong weld. Measure from various reference points to be sure the dimensions are right before welding. Trial fitting the old windshield would be a good way to check the fit while doing the work.

You could go to www.transamcountry.com and do a search of the forums for roof swaps or just ask your question there for advice. I've also been doing a Google search for roof swapping and found some info at www.nastyz28.com , a camaro site.  

Also, here's a very good thread I found at transamcountry.com . http://transamcountry.com/community/index.php?topic=401.0

As for the structure of a t-top roof versus a hardtop roof, everyone will tell you the hardtop is stronger for obvious reasons. A Fisher t-top roof being a factory roof is certainly strong enough for normal use. For higher performance use, it is recommended to install sub-frame connectors under the car to add rigidity.