Shorten and re-thread Rem. 870 shotgun barrel... good idea?

[rockland]

I have a Remington 870 Express Magnum 12 ga. with 28" ventrib interchoke barrel. I would like to have it shortened to about 19" (just in front of one of the ventrib posts), reinstall the front bead and re-thread barrel for the Remchokes. Gun would be used with buckshot in scrub and regen bush.

Anyone done this? Is there even enough thickness in the barrel wall to permit cutting the internal thread? It's a left hand shotgun and spare/replacement barrels are unavailable, so I don't want to risk wrecking it!

Any advice appreciated.