Never fitted Saab seats, but have done shitroen bx and currently recaro all I can say is fit the ones with the lowest height from runners to top of base or you'll get annoyed with keep hitting your legs on the steering wheel when you get in !
Just drill and bolt through the tubs to fix with the biggest bolts and washers possible or weld the runners in ? Up to you..
Yes, SAAB seats are too big all round imo. I have fitted Metro seats as they are a similar size to original but with a bit more support/comfort. They were £10 each from the breakers (they also have flat runners so are a relatively easy fit)
I have white leather saab seats from a 9000 runners bolt straight in I just drilled 4 holes the other 4 lined up. and lots of adjustment. Heated too if I ever wire them up.