Here's what you need, they work great on my '65 Sheridan as well as my late model Benjamin.
http://centerpoint.crosman.com/accessories/B272
http://www.pyramydair.com/s/a/Crosman_B272_4_Pc_Intermount_3_8_Dovetail/191
You might also check out the INTEGRABASE.
I plink, therefore I am.