These are pre-threaded and long chambered,you do not need a chambering reamer unless you "miss"
Careful depth mic work ,rec ring to diaphram,rec ring to bolt face,"go" gage protrusion from the chamber of the bbl,make a sketch.You want a little crush on the end of the barrel,to the diaphram,I shoot for .001 to .002.Others may have advise.
I have depth mics,and the Starret mic you can change the fixed anvil...multi-mic? the kind you can put a pin in for neck thickness.Well,I take the clamp clear off and it becomes a small height mic.Works good to measure the headspace gage protrusion from the chamber.
If you prefer not to just boldly cut to numbers,remember you can use shims or feeler gage like if you use a .005 shim between the bbl end and the diaphram,try the go/nogo.If the go just goes and the no go won't,you have .005 to take off yet.If the nogo goes,take the shim out,gage again.You might be done,etc.
I have one of the Walther 718 contour bbls,very light,about a #2 Douglas.Threads,a fine snug fit,all finish,beautiful.
They are listed at $226,threaded,excellent exterior finish,long chambered.Does not seem expensive to me.
I have never used a McGowen barrel,but Harry has been making them a long time,seems 40 years,anyway.I was looking at a .405 WCF blank on his page,he'll custom contour,octagon..
Generally,if you do not make a good product,you won't last 40 years.