Richard,
I've done a few of these problem childs...  and I've come up with this fixture to single point threads in castings and repair/chase existing problems.   
For as many miles of threads I cut in my day to day work, this isn't a real big thing.  I don't do this type of work for hire but here's a picture of one of the actions I salvaged, it was a LW that I welded up the several different areas and re-machined.  Probably should have just tossed it for the effort required.

Photo f3 is of a Farrow that had the threads tapped, they weren't square with the world.  It has a shank stuffed in it to get it close when I went to the lathe and I'd mapped it out so I had an idea how it needed to be dialed in to square things up with the breech block.  I chased the threads, truing them up to the face, even though they weren't a standard size, I made the barrel and fit it.  It turned out well, torqued up square and everything was fine.
   
If the threads in your action are sound, turn a plug that fits the threads to a snug/tight fit, leave several inches of shank (turned true at the same time as when you cut your threads) to dial your action in, make this simple fixture, and dial to the shank.  From here you can take a light skin on the face of the action to put it square to the threads, easy peasy...

Maybe 5¢ worth of info?
Greg

Square with the breech block would also be a consideration.

I am greatly impressed with that fixture!

Charlie Dells book about schuetzen rifles has the easiest way I've seen to square reciever face and block to threads. 
A threaded bushing, theaded OD, theads nice snug fit to reciever.
ID bored true to threads for a pilot hole. 
2 laps, one for the BB, one for the face, piloting in the bushing.
Drive laps with a hand drill. 

Tomorrow, while my wife is watching the Cat-Griz game I will see if I can find one I made years ago and post a picture.

Man that spell check could screw up a steel ball with a rubber hammer.
Kootne, I remembered I have Schwartz and Dell’s Schuetzen Rifle book and read the part you talked about. I don’t have any cast iron for a lap but do have some 1.5 OD brass rod that may make a descent lap, what do you think?
Thanks Richard

Brings back memories of 22 1886 Winchesters that were sent to my shop for re-barrel. The actor Brad Johnson thought himself a gunsmith and took about half the actions to a belt sander to grind the lip off for making the rifle takedown models. None of these were square to the threads when he was done. I screwed a barrel into the acton and dialed it in on the mill and faced the action. Luckily for me  the frame extensions were new and had never been fitted to an action so each one could be fitted to a trued up action. Some were done in 45-70 and some in 50-110. I never got to see the finished product but did hear a few stories from the stock maker that did the stocks for many of the rifles. Tom Selleck ended up with one of them.
If I were doing a low wall or high wall I would be using the mill. A more rigged setup could be made off the edge of the table.

Now that I'm Cafinated one more "Think About It". With the Breach Block Wedged into position by the Stub Arbor then held in position by the Cerro Safe could one use a Boaring Bar to true up the Breach Block Face?  Come to think about it I have a Low Wall that needs work.  Winter Project 😲. David

Made a threaded stub arbor and it worked fine. I only had to take of .007”