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You will need first to contact Llija and see if they do .577 or .600 barrels. I know lother walther do 577 I am not sure from memory about Llija, but I dont think they list them. Pac Nor do. Krieger do.
Personaly I think that the shoe lump will be fine. A lot has to do with fit.
Before you begin you need to contact a firm that can do vaccum/nitrogen brazing. This should not be done open hearth because of the possibility of 500 degreeF embrittlment of 4140CrMo. Also it will protect the rifling and the chamber as there is no scaleing. Do not let somebody talk you into open hearth !!
When you have spoken to them they will talk to you about brazing alloys. If you talk to the barrel manufacturers they will tell you what degreeF the barrels were tempered at. You will not want to braze at a temp higher than this as it will effect the properties of the barrel.
Whilst you are talking to them they will be able to tell you what clearences they need for brazing (this varies with alloy)Some are very forgiving .005 is common.
There is tremendous strength in a properly brazed article. Many other things would fail before the barrels detached from the shoe.

When machineing the lumps one must be as precise as possible for strength allowing only such clearence as they recommend. You may need to make a mild steel brazing jig to hold this lot together for them.

You need to plan the quater rib and crossbolt extension as it will have to be brazed together with the barrels and lumps.

The Hyem one looks different because I think that you would find the C-C of the firing pins is less on theirs.If you moved those barrels apart it would look like yours.

Whilst monoblocking is fine for a working gun (my own gun is done that way.) Taking the time to make lumps will produce a much finer finished article.

Try to work in one plane only at a time . i.e. set up in the mill for a vertical fly cut of the barrel beds. Without moving the set up drill for ejector rods. Now you can move the job 90 degrees for the action bed and lump cuts and all should be perfectly perpendicular.

Whilst you have the macine set I would make 2 of everything. The materials are not that expensive and if there is a screw up at any point then you only need to go back one step. If nothing goes wrong then it is a good excuse sometime in the future to get a set of barrels in another caliber.
The point is the set up takes the time not the cuts.

All the best

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