Ok, so I am going to take a block of 7075 aluminum and bore 2 holes about 6" apart. Then the openings need to be female threaded to accept threaded male gland nuts to hold stuff in.

How does one make such large threads into tehe openings of holes> I know I can use a four jaw chuck and offset each hole till it indicates on center and then thread it internally with a boring bar with a thread insert, but is there a way to do it on a vertical mill?

It seems UNC or UNF taps that size are next to impossible to come by and make be really expensive if they did make them.