1)There are various kits out there. I've personally been looking at the Baer kit: http://www.baer.com/Media/ContentDetails.aspx?ContentID=107
but it's expensive. You can do a stock GM conversion on the front:
http://www.firstgenfirebird.org/firebird/FAQ/wheel_brake_axle/brakes.html

2)I once saw a website that had this exact conversion for 1st Gen Firebirds but I can't find it now.

3)Firebirds came with power windows as an option, not sure on the power locks though. If you used "stock" parts for the power windows conversion it would be expensive.

4)I run the 350 in my 1968 Firebird, at the bottom of this post you can click on the link and it has a full rundown of what I've done to my engine. Converting to a 4 barrel would be a big step in the right direction.

5)My personal favorite is Ames Performance:
http://www.amesperf.com/
they always seem to have the right price and the part actually fits when you get it. I've had fitment problems with Classic Industries:
http://www.classicindustries.com/
in the past (no a Firebird is not a Camaro especially when dealing with the engine/trans) because they try to sale Camaro parts for the Firebird. There's National Parts Depot (NPD):
http://www.npdlink.com/
which I haven't used but they have a good selection, Year One:
http://www.yearone.com/
is expensive when it comes to their resto parts and they don't have much of a selection compared to the other Pontiac specialists, and last but not least The Paddock:
http://www.paddockparts.com/Paddock/assets/html/homepage.asp?URLCheck=1
again they tend to sale Camaro parts for the Firebird so be careful.

All of these companies have PDF files you can download that have their latest catalog. Or you can contact them to get a hard copy of their catalogs. They all have websites also.

Hope this helps you get pointed in the right direction.

G.J.
68 350 Coupe