2nd gen Firebird alignment
 

Hey, I’ve got a 72 Firebird. The car currently has lowered/stiffer front springs and delrin bushes in factory control arms. I’m wanting to put stock type springs and rubber bushes back in to improve ride and nvh. I’m thinking of trying to gain as much castor as possible. If I use offset upper control arm shafts, installed so the control arms are offset towards the centre of the car, which will limit the amount of shims needed to gain 0* camber then could I use a lot of shims on the rear to gain maximum castor?

Yes, that’s how that would work. However you’re still unlikely to get a “lot” of positive caster out of factory control arms.

You will be able to achieve some more than typical, but I wouldn’t expect more than about 3* in this type of configuration.

Considering the offset control arm shafts and tall ball joints will cost $240 plus bushes am I better off buying bmr upper arms that have powder coating defects for $335? The bmr arm have 2 degrees more positive castor and more negative camber built into the arm. The only downside I see is that they have poly bushes and I wanted to go back to rubber for a better ride.
How much difference in NVH would there be between poly and rubber uppers if the lowers and rubber in both cases?

I agree even with offset shaft and factory A arms best you are going to get is 3degrees positive caster, then if you want some negative camber often it drops down.
I ran sold bushings in my 78 TA and it was not harsh.

Agree with SkipFix here, your ride issue is likely less to do with your current delrin bushing and more to do with the spring and shock that's in the car.

Try the RideTech progressive rate spring and shock if you like a slightly lowered stance but are concerned with ride quality.

In fact, before I made a whole bunch of changes, I'd probably try a different shock first.

Shocks are new koni red. Are these known to be harsh? Springs are to stiff for a street driven car that gets out 6-8 times a year. Steering feel and ride comfort is my goal, not best handling

Depending on how they are setup, they can be harsh. The Koni red's are adjustable and you can take some compression and rebound out if the ride is feeling too harsh. For the classic red's the adjustment must be made with the shock out of the car.

Here's their how-to.
https://www.youtube.com/watch?v=u_eg9Kwv9NM