Jon, sorry this may be redundant, but would these headers fit a 1980 900t with B engine, and what kind of difference in performance would it make?

Sam, turbo headers and non-turbo headers are completely different; it won't fit your turbo. I have never seen a custom header on an 8-valve turbo, not saying someone hasn't fabricated it, I just haven't seen one, yet. I have thought about building one but it is very low on my priority list.
I have seen an 8-valve H engine turbo exhaust manifold and turbo on a B engine.

Cool thanks Jon. Neat pic of the H manifold hooked up!

Found a new set of these (with exhaust) for sale @ the SOC for $350 .. sounds like that's a good deal

edit: [change in plans]

SalemSaab wrote:1.5 years later. Still trying to get these installed. Gave some cash & all the parts & hardware to my guru 6-7 weeks ago but getting it back uninstalled.

Anyone know of a place that will install headers & exhaust at a reasonable rate in Oregon?.

Gotta fix my 900T too but really want this done, just dont have shop-rate $$ for someone who has never seen a 99 before.

I should have the install bits & instructions if my guy didn't lose them.

Also ISO a better setup for the 16VT but that is a lower priority .

I hope you do not take this wrong.
You own 2 classic Saabs, not a lot of spare money, and want to keep them running and/or projects?
A cursory review of this thread seems to me you have a full bolt on MSS header and exhaust for a 99 B motor.
For starters, if you are in fact selling the 99, do not install it, but present it to the potential buyer as an option or a item that will seal the deal. If the buyer does not want it re-sell it.
DIY, exhaust is a perfect bolt on project. Toughest part is pulling off the old exhaust manifold. Some of the studs may be challenging but some good soaking with Krolloil, light taping and time, repeat, should come right off. In my experience my B motor bolts were always loose. You will need a new gasket of course.
If there is welding involved, rig it up so you can get it to a local exhaust shop. Exhaust is pretty much exhaust. They know they need room for suspension travel, body knock, and soft mounts to keep from future breakage.

Don't have a garage? Buy some things to make it nicer. Big blue tarp to put on the ground and park the car over it. Trouble light works wonders. 6 point sockets. Beer can be helpful. Check the forecast and look for a nice weekend.