Is there a trick I should know. When you cut off excessive threading with a hacksaw
Will the nut still turn on the thread. Because now you have a rough finish.

One thing you can do. When you cut off the threaded pipe, do so with the nut screwed on above the point where you will cut. When you're finished cutting, then simply screw the nut off. When you do that, it smooths the cut and you can put the nut back on easily.

Jlisenbe is correct. Also, if you have a bench grinder you can grind a taper on the end of the pipe to clean up the cut.

I never had much luck grinding a taper . It always leaves a little burr. Short of a set of dyes to remove the burr left by the cut jlisenbe gave you the only option there is. Good luck, Tom