Hell0 Gabor
Rectangular flanges have their own special problems, and are not included in the various codes (as far as I know). For a rectangulare flange to hold pressure, it must be designed as a flat faced type flange. The gasket must go to the edge of the flange. The flange is designed as a cantilever. This is an entirely different analysis from the one set out in ASME Section VIII, Division 1, Appendix 2.
With regard to your second question the suggestion is a good one. However there seems to be no difinitive method of calculating the required torque. Many engineering corporations have their own internal standards, and there seems to be no universal agreement even in regard to published methods - for example, in the Pressure Vessel Design Manual by Dennis Moss.
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Sincerely,
Ray Delaforce
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Hexagon PPM