In my opinon heim joint are the way to go..... specially if the shock will be pretty stiff !
Any gas shock can be mounted up side down..... twin-tube shocks (like QA1 for example) cannot be istalled up side down.
In fact a gas shock must have a floating pistone inside the body to separate the gas from the oil, thanks to this you can do that.
A good reason (in light cars) for an up side down mounted shock will be the reduction of the unsprung mass.
In our cars the unsprung mass is so big that it will make very little sens.