Well ,it will make 400 horses pretty easy.But it wont be correct for a GTO .
The 63 back 389 used a bell housing mounted starter , making it a bit harder to run a decent trans.
It would be OK for an older poncho like 63 and back.
I would look around, shop around, ask questions.
You have done the first thing you should do when pursuing a build. You have decided to build a street machine/daily car, with around 400 horses. Totally do-able, and sure to be loads of fun.
So , for a street machine, you need to get a car. That will be your next choice.THEN look for an engine to build, as I said some wont bolt in or work correctly.
A 400 is an awesome engine, a 389 is also good but remember what I said about motor mount bosses(bolt holes)
You wont find a 389 with the correct bolt bosses for an F body (firebird-T/A ) so dont even bother. And the 400 was built in 1967-up. But the 400 wont have the extra bolt bosses for an F body either, unless you get a 70 or later block. It pays to shop, but get schooled a little first.
Do some looking at block casting numbers and date codes and such.
http//:Wallace Racing-Drag Racing Pontiac powered Firebirds,Trans Ams and Dragsters
is a good place to start with some basic info. They have a lot of tech info that you will want to learn about.Like how to ID a block from the casting number, where to look, how to date it, by date code and how to decipher... lotys of good reading over there.
I will post a few others for you too. Later