I actually had a similar plan. I like the look of Targa tops. It is a large undertaking. You have to redesign the A-pillar to have an "exo-skeleton" that is a front roll loop with the structural triangulation under the dash. Same has to be done on the B-pillar and a structural bar across the roof between B-pillars. Once those are built then build the extra structure across the floor in the door opening. If you don't start with a T-top car you are crazy because a T-top already has the top structure you would need for the Targa part.
It was not the design and work to do the conversion that changed my mind. What you may not think about is the size, weight, and awkwardness of the Targa roof that would be created on a G-body. It is a huge piece to pull out by one person and then where to put it? Hell T-tops take up most of the trunk as is and they sit on top of each other.