Ithinkverydeeply,i must apologise beforehand for what i'm about to say,i absolutely don't men to come off as some self-appointed anvil-nazi...
...
But what is shown in photos above is a pretty bad idea.Again,what an anvil Does,as a tool,is it transmits energy.One can think of it in terms of electrical current,or any other energy flow,as in any inconsistency messes it up fairly badly.
To actually accomplish what's pictured in the photos above the weld joining the plate to the rail section would have to be 100% solid(the second photo makes it look rather doubtful that it was achieved,or even planned).
Such a weld would require a Lot of time,skill,rod,and electricity,and then such a critter would still need to be heat-treated.
Both the heat energy and the set-up for such project(quenching situation especially)would be pretty challenging...
All of the above together puts the project Way out of proportion with any practicality...
...As an expert craftsman yourself,surely you can relate...
I do hate to be such Debbie-downer,and so write this solely in the hopes of saving someone a lot of hassle in pursuit of a romantic misconception.
On the positive side,if one absolutely cannot get a hold of an anvil(that are not That tough to come by),a good practical substitute would be as heavy of a tool-head as one can find.
A 10-12-lb sledge would be already pretty much ideal...(bigger-fantastic,and they do(or did)come in poundage up to what,20#?).
Such head,nested a short way into a stump,would be pretty much a real working anvil.
I've not kept up with box-store sledge prices,but i'd not be surprised to find something totally useful for under $20...
Btw,a #10 sledge-head in a stump would amount to about Twice what many an early Middle Ages smiths used as production tooling...Our museums are full of beautiful(and fairly massive) tools and weapons all built on +/- #5 anvils...
The last couple-three centuries of Industrial Age with their machine-building requirements have massively skewed our view on anvils...