Where do you drain the engine oil for 1986 Honda 350 fourtrax 4x4?
into a form of containment unit
It will depend upon whether you mean the engine oil drain or the Primary/Transmission oil drain. The oil drain for the engine oil is actually NOT a plug. Underneath the engine is a hose that is clamped to a boss on the frame. Loosening the worm clamp and pulling the hose off the boss will cause the oil to dump (you did put the drain pan under there, didn't you?)
My service manual states: 2.6 US qt. for manual transmission with oil drain 3.2 US qt. for manual transmission with engine rebuild 2.6 US qt. for automatic transmmission with oil drain 3.5 US qt for automatic transmission with engine rebuild I believe only the Hondamatic has an automatic transmission so that info probably does not apply for your bike.