Florence Knoll sofa model 67a upholstery help needed....  

I'd love to see a pic of the...
I'd love to see a pic of the sofa and fabric. Picked up a Flo Knoll loveseat myself recently.

Here are a couple pics of min...
Here are a couple pics of mine. The pics aren't my actual sofa but the same model. For the fabric I'm using Maharam Hallingdal 126.

Oooo!
Did you get that Hallingdal that was on Ebay for under $20/yd? I would have been all over that if it had been one of their other grays. I noticed it got snapped up very quickly!
As for foam--is there any way you can test out different densities on the sofa itself? Is the webbing ok? It may very well need to be replaced, and that will affect the feel of whatever foam you put on there.
The original had latex foam, which is still available but costs more than urethane foam. It also comes in different densities. It's such a personal thing---some people like firm, some like a softer feel that you can sink into. Probably a firmer foam will work best with the clean lines and tufting on this sofa but within that there is still some wiggle room as to comfort.
The firmest urethane foam is extremely firm, more like what you'd want for one of those ottomans that serves as a coffee table. I would really try to test out at least two different densities if your upholsterer can accommodate you.
Also, the back cushions should be at least one density level softer than the seats. If the same, the back will feel too hard.
Good luck!

You can buy latex foam at
foamsource.com or foamorder.com in custom dimensions, but you still need to know what density will feel best. I used 3" firm (I think) on a vintage Knoll office chair once over tightly stretched elastic webbing and it was the most comfortable chair I have ever sat in, pretty much! Latex has a different feel than urethane---more resilient, more bouncy. It doesn't develop hollows over time. It's pricey, though---3 or 4 times the cost of urethane.
I would seriously consider it for a sofa like this. Authenticity is important but authenticity plus comfort kind of trumps all else.

Sorry for the belated respons...
Sorry for the belated response Spanky. That was me on Ebay. That was the killer deal that I've been waiting for lol! I'm gonna use latex foam on the sofa I think. Probably a medium firmness on the seat bottom and medium soft on the seat backs. I'll also be reupholstering a matching pair of slipper chairs that I purchased with the sofa. I'll have to post pics when I get everything done. Thanks a lot for all your advice it was a big help.

MCManiac
Congrats on the Hallingdal snag. That was an incredible deal!
Do you have a lot of experience with upholstering? That sofa requires a lot of precision work and the welt will be a bit tricky without an industrial walking foot machine. Upside: Hallingdal is dreamy to work with!