Nissan did make several vehicles concurrent with the 240 that bore T-tops - the NX and Z32 series - so it shouldn't be terribly expensive as far as parts go, provided they will fit the 240 roof profile. I know that Camaro T-tops run about $300 brand-new each, but they are pretty large. You can probably get a set of older NX tops for that full amount from a junkyard.
Then it's a question of having a shop cut out the roofline and reinforce the center bar, installing the appropriate seals and pin locks, etc. There will be a significant amount of labor involved, for sure.
There's a couple of places way up here in NJ that work exclusively on convertible conversions and sunroof installations, I would check around in ATL and call some of those places, the yellow book should have a separate section for auto sunroofs. Sorry I can't offer you more detailed info, but that's a pretty rare conversion. More power to you, though. T-tops rock.
