You can prolly find the spec in the Briggs Antique service man. Download a copy here:
http://bgsengineclinic.com/downloads/Scroll down the list, you'll see it.
3/8-16 gr.5 cap screws into cast iron/steel should be 35 ft.lb. more or less.
Personally, I'd use some sealer on the gasket and just get em' tight. No big deal. You wanna make sure both gask. surfaces are free of burrs, etc.