Something that bugs me is why you're keeping as much of the Firebird's structure as you are?
Looks like a fairly large issue is the height. Top of the Prius cowl area, bottom of the windshield is the fixed point to work from. How hard would it be to reshape the area where the bottom of the Prius windshield was to fit the bottom of the Firebird windshield?
Were it my project, I'd be 'skinning' the Firebird and grafting the outer panels onto the Prius, shaping and welding in filler pieces of sheet metal as needed. Either the Prius seats could be kept or I'd build adaptions for the Firebird seats to the Prius mounts.
Ho does the Prius door sill height above its floor compare to the Firebird? A common tactic in lowering custom cars is "channeling", which often results in the bottoms of the doors being raised to fit over the top of the frame while the outside of the door extends farther down so it looks normal when closed.
Whatever you do with it. It only takes time and money to get it how you want.
I bet with enough 'massaging' of the Prius parts you could get it all stuffed within the stock body length of the Firebird.