Try to use the ASME B&PVC Section VIII Div. 1 Appendix 2 there you can find the rules to design integral non metal to metal contact flanges.
Best Regards,
DFGD
The use of stud bolts is accepted, you have to use only two nuts and two plain washers, but you have to thighten it with a torquimether (In some mechanical handbooks you can find the proper torque). Also for safety you can use thread lockers (for reference see loctite web page) or lock washers...