I posted this image over on HGN (http://hydra-glide.net) for a builders reference. This is an OEM cover and OEM position of the felts. The oblong holes should be directly over the rocker bearings. Using thin gaskets will allow the rocker arm bearings to make contact with the felts (depending on the actual depth of the repop covers. Covers vary from mfgr. to mfgr.) Note the rocker bearing nut indentions in the felt, around the oblong perimeter edges.
I use thick corks and (black) 3M weatherstripping to glue the felts to the cover. This application of black 3M ? is a little too sloppy for my liking. Given that the felts get destroyed if you try to remove them, and the presence of the two small factory embossed circles on this felt, I doubt these were ever removed. Doesn't look like a what you would see as a factory application of glue today though. But, the 3M weatherstripping is impervious to oil, so maybe the factory knew the glue wouldn't de-lam. and get run through the motor and didn't care if it spread-smeared. They used glue of some sort, even if 3M weatherstripping wasn't invented in 1948.