The Chevrolet Shop Manual does give torque values for the oil pan (6-1/2 to 7 ft-lb for the flange screws and 12-1/2 to 15 ft-lb for the corner screws), but not for the two valve cover nuts. I'm sure that they do not need to be torqued as tight (25-30 ft-lb) as the stud nuts for the rocker arm shaft supports.

Most published tables for torque values are for metal-to-metal or hard-gasketed joints (like a head gasket); most machine design books say that the torque for a joint with soft gaskets needs to be determined experimentaly (i.e., find out what works). I've had good success with the methods that Tim and Jerry describe.

Hoyt