There have been reports of poor ground clearance with TA headers. Be prepared to heat/bend them to get the collectors higher.
I'm using a set from Kustom and ground clearance isn't very good..... gotta be real careful over speedbumps.
1-5/8" is common on the primaries, collector sizes varied from 3" to 2-1/2" on the later GSCA repos.
HP increase is likely exagerated. The stock manifolds are said to work pretty well.
Brian had headers on his 4 speed car. There was some interference, maybe he'll respond here with details.