I'd say if you want a SS Pontiac, GT is the way to go. You can find a good buy on a used GM SS roller, then put your Pontiac engine in it.
https://www.racingjunk.com/Pro-Stock...89&from=search
Or, if you can live with a '77 350, you can probably get by with a good Stocker roller. The '77 350 hp factor is only 250, in GT. I think Larry Maxwell is running 9" slicks on his '74 350 powered '68 Bird.
And there are lots of Stockers running low 10's on 9" slicks & legal Stock suspension. Should be lots of 3rd gen Stock & SS rollers for sale out there, especially when the season is over. Should be able to find a Stocker roller for a reasonable price.
If you wanna stick with an older Pontiac body, for GT, you can use an X-body that didn't come with a Pontiac motor. I assume that will include anything form '71 thru at least '79, & would also include a Phoenix.
And, since the engine & body don't have to match, you can use a '79-'81 Bird body. I think more '79 Birds were sold than any other year. So, it makes sense that they will be more plentiful & cheaper. Have also noticed that the '80 & '81 Birds are usually pretty cheap. I personally don't like the '79-'81 nose. But hey, looks ain't everything. And, I know the nose can be replaced with a '77-'78 nose. I'm assuming the tail can also be replaced with the earlier tail, so that one could run it as a '77-'78.
Anyhow, for GT, there are LOTS of body options.