I got one of those evil flyers in from J&G Sales, and they had Saiga .410s at $299. I had a brief moment of insanity before slipping my credit card back into my wallet...
So, there's a brand-new Saiga .410 laying on the bench. And since there were some .45-70 cartridges laying around, the Voices got all excited, despite the fact they've been no help finishing off the dozen-odd projects I've already started. And there's a spare, chambered .45-70 barrel leaning against the wall.
The .45-70 will almost go into the Saiga mag. The Saiga bolt would require welding some extra metal on the fixed extractor hook - it has two, one fixed, one spring-loaded. The problem is the GIANT RIM on the .45-70. But a little welding, a little filing, a little more filing in the plastic magazine rim slots, and replace the barrel, and I don't see why .45-70 wouldn't work. Well, except for cutting off the bottom of the gas block and welding a bigger tube to it, but that's no big deal.
In fact, it looks like a lot less work than Willie's .500 S&W conversion.
.444 Marlin ought to be easier, but I can't find any .444 brass in the box at the moment.
Presented FYI. I'm not going to do this. I have way more than enough projects going already.