Have a small leak in the bathroom and I need advice on. Just refinished the bathroom so ripping out and replacing all the pipe is not an option

We have old 3/8" iron pipe supply lines throughout the house.

The leak it where iron pipe meets the shutoff valve and I still can't manager to nip it.

The old iron pipe has a rusty connection end with bad threads. No amount of Teflon tape, plumbers putty, plumbers epoxy, O-rings, or similar seems to help.

My next plan of attack is to just add more threading onto the existing pipe, then cut off the bad section. (I have 6-8 inches of pipe to work with on the finished side of the wall)

However I can't seem to find a proper die set to actually add more threads.

The true O.D. of a 3/8 Iron Pipe is .675 inches which is not a standard die size.

Would using a 5/8 die be detrimental? .625" vs .675" so .05 inches difference - should both be 18 threads per inch...

Any Advice?