Gauge cluster can be done, but its quite painful and you need to purchase the correct senders. I have done it, and probably would not do it again. Looks cool, but idiot lights function as well. I think I have an xtra couple of sets of '68 rally gauges I may sell some day. ('67 and '68 are not the same)

I have done three rear fold down conversions now, and sold two additional ones. Keep in mind that '67 is different than '68-'69, but will interchange if being 'correct' does not matter to you. Seat covers for fold downs are available, but are not the same as non fold down covers. Some of the fold down parts are available, but not all. You need most of a donor seat and trim from a fgf or 'Maro. I would call this a 'very cool but completely useless' option. In my experience, '68-'69 seats are way harder to find than '67. Not sure why, as there were less '67's built.

"Most" firebirds came with manual tops. I don't know actual numbers, but in my experience, I'd say 10% - 20% tops were power. (pun intended)

Good luck!