Shiff, great score on the
436.---I have that same vise and wouldn't take a gold monkey for it.---Rarely see them come up for sale anymore,---Used to pop up right often on EBay and CL but I haven't seen one in a couple of years.---Bad about the nut cracking but glass half full, if it hadn't your vise might still be in use and wouldn't look nothing like it does today.---Other than the nut, excellent shape.
Pretty sure the nut is cast iron so I'm not sure how well 7018s will marry up to cast.---You being certified, I figure you probably know what melds best with cast.---You are right about not going all the way through to the threads with your V-grind because cleaning those threads from the inside will be a bear.---Would probably have to go to a machine shop and put on a jig and recut/chase the threads.---Some will prescribe brazing which is a pretty substantial fix, but won't quite have that original look so to speak.---That wouldn't bother some, but wouldn't let others sleep at night knowing it was there.---Just depends on the person.
A more expensive but more natural looking fix is
Eutalloy welding, or
Eutalloy-powder-spray-fusing.---they're the same thing.---I've welded manifolds and cast iron cooking pots and cast iron farm implements with it.---It uses cast powder that blows into a molten puddle of iron, becomes molten itself and fills up the gap.---It's a pure weld, strong and neat, but as I said it can be expensive getting started.---Once you've got all your stuff then all you have to buy is the powder from time to time.---You probably already know about this stuff but in case you haven't seen any of it, here's a EBay link to some examples.
https://www.ebay.com/sch/i.html?_od...ly.TRS0&_nkw=EUTALLOY+WELDING+supply&_sacat=0