Some time ago my partner printed some wedding invitations on a beautiful soft stock that showed off a blind embossed image really well. The paper seemed thicker than Lettra 300gsm/110 lb, but not as dense/rigid as Lettra 600gsm/220lb Lettra. She thought it might have been Coventry Rag 320 or 335 gsm, but I do not see the Coventry Rag paper on your list though I thought it was produced by Legion. Is there a reason you do not carry? Is there any other stock that you would recommend that is thicker than Lettra 300gsm, but not stiff?
We would never leave Coventry off any list that includes any intaglio or relief process!
Here are papers for letterpress: http://goo.gl/qQ0m9v
Here are papers by name: http://goo.gl/y2ovlX
And here's the Coventry page itself: http://goo.gl/cGdsMi
It is very much still available! Click the where to buy link on the Coventry page to find a reseller near you.
I hope this helps. Happy printing.