The only way you will get a 427 cobra is to buy second hand or to drop one in after it is regoed. Doing the later will make the car technically not road worthy which is fine if you don't get caught but insurance may be a problem.
So if you really want a 427, the second hand mkt is for you.
If you want to build there are many manufacturers to choose from.
In Melb you have (from most to least $$) Robnell, Superformance, Python & RCM. From hearsay both Robnell & Python have slowed down production - not sure exactly what that means but you should find out. Superformance is turnkey minus only (everything except engine, transmission & wheels is done) b/c they are imprted from South Africa, so that pushes up the price. Robnell is really good also, using carbonfibre in their bodies etc but they also engineer alot of there own parts so getting cheap doner parts is limited so that's why they are the most $$. RCM's are one of if not the cheapest kits to build b/c most of the kit is from donor parts. It's a hands on kit, requiring you to do most of work eg hanging doors, boot, bonnet preping the body etc so that's why it's cheap. I don't have a trade background and so far I have found it fairly straight forward to do but I have had to learn ALOT very quickly. Laurie Bingailas (RCM) is very helpful and will guide you through the build. Expect to shell out $30-40K depending on what you can do yourself. Also ADR 79/00 requires an engine to be almost new so that has pushed the build price up.
Don't know too much detail about other kits in other states but I would have a look at Classic Revival in Sydney or ask their unnofficial agent in Vic Aussie Mike (Mike Murphy)
If building choices of engines is now limited to Gen3 or Boss 5.4 - Both of these motors can generate serious HP but the Gen3 is becoming more popular with builders b/c of light weight and packaging. Boss is three feet wide across the beam & most manufactures still haven't figured out how to fit one yet. But when they do there are bound to be some compromises b/c a cobra just ain't big enough to fit one.
Whatever you do, ask lots of questions and feel free to come along to the Vic Cobra Club so you can talk to like minded folk
Cheers