I overmat my prints with a 1/2 inch white border of the paper showing, and sign and number the print in that border. For the Chrystal Archive, the Micron and Staedtler pens, which feature archival pigment inks work well. I favor the 0.1 point to make the signature as inobstrusive as possible. For non-photo papers (as in inkjet), those pens don't track smoothly on the paper surface, so I find the Sakura Gelly Roller, which also features pigment inks, a nice alternative.

If you are uncertain, I suggest you take a scrap of the paper you plan to use to your local art supply store and try out the various pigment (that's what you want for achivability) ink pens to see which performs the best.