[some of this I posted elsewhere, with regard to .45-70 Enfield conversions, but Saiga mags can be made to fit AKs too.]
A .45-70 will NOT fit into a .Saiga .410 magazine. The rim is too large. It might be possible to disassemble the magazine and (with patience) open up the rim track with some riffler or jeweler's files, or maybe a scraper tool.
Just for kicks I tried a different Saiga-410 magazine and... almost!
The rim drops down into the feed groove on this one, but it won't go down into the mag. The rim hits the corners of the steel feed lips. If you trim the corners back a hair, it should go.
I took the magazine apart and tried loading it from the bottom. There are tight and loose spots where the rims ride. I imagine a little scraping or filing in there, and you'd be able to get them to feed .45-70.
Like the .410, the noses of the cartridges would ride very high, and they're only retained by the feed lips, which would have to be shortened, unless you carefully carved into the plastic guide grooves... hmm. Anyway, it looks like you'd still need a Saiga-style barrel extension to keep the cartridge from simply flying out of the ejection port.
I wondered what I was going to do with the two Saiga-.410s I bought a few years ago...
BTW, the Gibbs/Navy Arms .45-70 Enfield magazines look very much like the Saigas, or vice versa. And the Gibbs *does* have feed problems. The one I borrowed would just wedge the cartridge nose into the breech unless you worked the bolt with authority, and it only liked to feed when the rifle was right-side-up.
.444 Marlin fits a standard .410 Saiga magazine just fine. Four rounds before the follower bottoms out; they strip out just fine.
According to legend on forum.saiga-12.com, only three rounds will properly feed if you do a .444 conversion. I don't see any problem in the magazine itself.
Some of the .45-70 Enfield guys just turn their rims down a bit so they'll slide through the "C-ring" of the receiver instead of machining the receiver. I wanted to run any out-of-the-box Wal-Mart .45-70 in mine. It seemed like a good idea at the time, anyway... the Enfield's extractor has an absurd amount of travel, and minor variations in rim diameter don't bother it a bit.
If you were to turn the rims down from .608 to .590 or so, the cartridges would probably still work just fine in anything chambered for a .45-70 to start with... and you wouldn't have to grind the AK locking lugs for clearance, or do "ship in a bottle" modifications to Saiga mags.
Stock mags are 4 rounds, long stick mags are 15 rounds, and there are 30 round drums. I have one of the drums, but my wife moved it and I'm still looking for it. I have hopes the drum may "just work."
Perge, scelus, mihi diem perficias.