I've also seen the pto cover tapped. My preference would be to braze/weld a fitting to the cover after it is removed from the transmission. Even if its tapped with fine thread the cover isn't very thick to provide very many threads.
You would hate life so much if it rattled loose and started leaking the syntorq out!
If the correct sender port (weld on) didn't come with the kit, a suitable brass unit can be sourced at most good hardware stores and home centers. Likewise a steel bushing could be used for stick/mig/wire welding etc.
I think the threads on your sender are sae, male pipe, etc. vs metric?
Good luck,
Andy