Tom,
Your writing is harder than hell for me to understand, exactly, what you are asking, but I will try to answer:

Review of questions:
You have a Spanish 10 gauge 3.5" magnum, and are wishing to do a conversion to a 12 gauge rifled double. As an alternative, you may convert it to a .577 2.75" BPE double.
You are also asking if you can eliminate the breech end shim, as used by Ellis Brown, as explained in his book on conversions.

I took class in gunsmithing school, from W. Ellis Brown, on the process of converting double shotguns to double rifles, prior to his writing book on same subject, now in its 2nd edition, a copy of which you said you have, so perhaps that will help in answering your questions. Further, I have done several conversions in various calibers.
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My response to your questions:
In writing his book about the double rifle conversions, Brown, of course, has written it intending the conversions to be in double rifle calibers; not in gauge gun "calibers," as you are intending.

If I understand you correctly, that shim at breech end that you are talking about is used for the first step in the process of regulating the double rifle, keeping the barrel spacing within specs. he mentions. It is used to space the barrels at that point, to make for ease in regulation process that will follow, as he explains in book. In book, he explains the purpose of that shim, and also explains that the monoblock must be bored parallel; not tapered, for regulation that will come later. For regulation, as long as you keep the barrels apart within the specifications he mentions, your regulation to follow later, will go much better. You could put that wedge further on down the barrel, or do whatever you must, to keep barrel spacing as he specifies. Since you want to convert to rifled shotgun barrels only, your barrel spacing will be less than it is for "normal" double rifle calibers---you will have to work that out during your regulation process, following essentially the same regulation process he explains in book.

That Spanish 10 ga. 3.5" mag. you will be using is probably much the same as ones I have used for conversions to full nitro .577s; they work well for that. Ones I used were the boxlock actions, with Greener crossbolts--some do not have Greener crossbolts, so I never used any of those. In my opinion, these big Spanish actions would be a bit of an "overkill" for a 12 bore gauge gun, and also an "overkill" for a .577 BPE, being more suitable for a full .577 nitro, though they will work for your purpose, but give a much heavier end product. I have a Charles Lancaster 12 bore gauge gun with action just ever so slightly larger than normal 12 gauge double shotgun, that is of perfect size for the sort of thing you intend doing. I also have same maker's double rifle in .577 3" BPE, with action sized ever so slightly larger than normal 12 gauge double too; neither of these is anywhere nearly as large or heavy as the Spanish 10 ga. mag. actions. I have a friend who owns an original British .577 nitro double, and he allowed me to carefully measure that action, comparing its size, overall, to these Spanish 10 ga. mag. actions, and I was surprised, and delighted to find that these Spanish actions are almost identical in size to his classic British .577 nitro, making them most suitable for conversion to .577 nitro express.

It sounds as if you intend having your friend make you a new, from scratch, monoblock, rather than your cutting the 10 ga. barrels off, to form a monoblock. It is much easier to cut off the barrels, and form a monoblock from existing barrels, but if you don't want to do that, a new monoblock is the way to go.