The $64 question we have all been struggling with for several years now.
I do not believe anyone has any blueprints....yet, but several ideas have been bashed around.
I was over in Pennock's a couple nights ago looking for this exact info and there is LOTS of info but again, not many detailed drawings at all. I have my own design, derived from the Automoda design of many years ago but I have not gotten to that stage of the build yet.
B & B has a top that is also based off the Automoda top but it also has a removable targa section as did the Automoda top.
The real Ferrari top is a hydraulic setup that folds up the front section of the roof and then pulls the pillars straight down behind the seats. There isn't really very much room available in the Fiero for this setup but a few have been looking at trying to do that. See FunnyWheels' "Going Topless" thread here for more on that one.
Pit Bull is also doing some top work right now as well. Look at his 355Hell(StartingOver) thread. I beleive that is where it is.
When over on Pennonck's I saw that a convertible thread was up and rujnning and one of th emembers over there is having someone in Georgia copy the mechanisms for the 355 but I don't know who that is. It is supposed to be available in the summer time frame.
That's all I can help you with. If you go through the design process and come up with something, we woudl all like to see it as well.