Gathumper

A cheep and excellent description of threading and chambering a barrel can be found in Gun Digest Book of Riflesmithing by Jack Mitchell.

I would respectfully suggest that starting with a block of 4130 or 40 and trying to convert that to a monoblock with a milling machine or monoblock existing lumps without any previous relevent experience, is like trying to fly a F15 using the manual.

IMHO get some 1"dia mild steel bar stock and build a pair of solid barrels with this first.
Drill a hole in the end and but a cheep used chamber reamer off e-bay or such. 45-70? perhaps and cut a couple of chambers.

Get a good set of files a hacksaw and some square and rectangular stock and duplicate something simple in solid ( a desk stapler is good) Look at the finished item critically. Is it an accurate and near perfect faxsimilie? If not, do it again. and again ...until it is. Now build a monoblock in the same material. Is it perfect for shap, size and fit? Good now go to the expensive stuff.
Aint got the patience for it? Then the real thing will be worse so its a non starter.

Practice soldering steel tubes inside other tubes. Take a tube 1" id 4 " long, turn a steel bar 4" long to a -.005 thou fit. Solder it 1/2 way in. let cool and stand tube don on a block. Take a 4lb hammer and try to punch the rod further into the tube. Tube buckle first? Rod doesnt move? Good.

Take your mild steel fake barrels and solder ribs on them after making and shaping them. Beat the hell out of the finished set with a rawhide mallet. All hold together. Good.

This isent me being a smart arse. I had inumerable problems when I built mine and I have been in this game for 28 years on and off. The only thing that saved my bacon on occasions is that I had performed all of the tasks before all be it not on guns.

Ellis Browns book is good, I have a copy here. However in many of his discriptions there is an assumption of experience. Back to the F15 analagy its like saying " After you,ve completed the negative 4g inverted pushover make an immelman turn" when you havent figured out how to turn the engine on.

Good luck.