I have built two 45-70s on the Siamese action. It isn't difficult but let me tell you right off, if you are an impatient or lazy starter non finisher, don't even get past thinking about it.
I am in no way suggesting that you are any of the above by the way.
First off, you can get a stock for it from Boyd's. Now, there is another slight curve. Not all Siamese Mauser actions are the same. Very slight differences between batches. If you're going to try to find a take off milsurp stock send me a PM.
Then you have to decide whether you want to scope the rifle or go with iron sights of one sort or another. The irons work well because you don't have to modify the bridge and receiver or drill and tap, unless of course you are mounting a micrometer type sight.
The bolt face will need to be opened and the extractor claw will need to be ground to fit in some cases.
You will have to source a barrel. There is more to this than meets the eye with such a build. You need to determine the twist rate by what sort of velocities you wish to achieve and which bullets you intend to shoot. Barrels will cost anywhere from $300 to $500 before taxes and shipping.
If you are looking for a very powerful mauler, this action is very capable and extremely strong, supposedly using the same quality steel the Japanese used in their Arisaka actions.
After my experiences with these fine actions, the next build I did on one would likely be in 458Win Mag just to take advantage of the stronger cases. The angled magwell lends itself well to this case as well.
I did see another where the smith used the existing barrel and made up a modified 458/8mm necked case. He had a 458 reamer on its last legs ground to make up the shouldered design. This was in Texas back in the 70s. That rifle kicked like a Mule with that light barrel but velocities were in the 8mm Rem Mag range and it was accurate. This fellow liked to play with such things and had the time/knowledge and equipment to do it well. His wife was a master engraver of both wood and metal.
The 45-70 can be stoked up beyond the endurance of most people so there aren't any flies on it either.
Remember, this is not a WEAK action. It is capable of accepting pressures of almost all modern cartridges that will fit in the magazine which is its only limit.
If you want it to be pretty, it will all need to be polished and blued.
In the end, it would likely be easier and cheaper to buy an already assemble rifle. The last one I sold a few years ago had a tough time bringing $600 in. It was in a well fitting light colored walnut stock, checkered, bedded and satin finished. It was drilled and tapped for a scope and wore a Lyman peep sight as well as a ramped blade front sight. Very nice rifle if I do say so. It shot better than I could hold with 405 grain jacketed and cast bullets. The barrel had a 1-15 twist and with 305 grain bullets it easily gave 2200 fps. The thing was, RECOIL was way beyond my endurance level at that velocity.
I shot a moose with one of the jacketed 405 grain flat point bullets loaded to the nuts at just over 100yds. The bullet hit him low just behind the leg, about 8 inches up from the chest bone. He just stood there shaking for about 10 seconds and collapsed in his tracks.
