I'm in the process of adding a basement bathroom adjacent to my laundry
room.

I'd like to insert a "T" into the 5" heat duct running along the floor
joist above my new bathroom, thus providing heat where there was none.

When all is said and done, I'll have a ceiling diffuser shooting heat
down into my new bathroom.

Question: how best to cut the existing heat duct to insert the "T"?

Would a reciprocating saw with metal blade do the trick? And can I
slice it in place, or am I going to have to pull the duct down to work
on it? (almost not worth it if I have to do that, in my opinion)

Never tried this before...any suggestions appreciated.