Turbo69bird used to make a nice kit for 67-69 firebirds. Not sure if he has any kits left.
Log manifolds should be just fine. Downpipe sizing depends on the horsepower goal whether or not it is a restriction.
I have a set of these on the shelf for when I convert to a single turbo. They are very nice quality
https://www.stainlessheaders.com/i-3...%26go%3DSearch