UPDATE:
I just acquired a 1905 35 WSL and a 1907 351 WSL. I'm going to post them on CGN and if they don't sell for a fair price, I'll probably convert one or the other.
Based on my calculations, the best starting point would be a 1905 32 WSL - but I don't have one. The bolt face would be just about right on. The mass in the fore-end would have to be lighted just a bit (like 10%). If the bore was good, you could even just bond-in a chamber insert and re-ream (and reload 7.62x25 with .311 bullets). The mag would be too narrow, but an M57/ TT replacement would do the trick.
Next easiest would be a 1905 35 WSL - and, as noted, I have one of those.
As noted, I also now have a 1907. The 1907 351 WSL would need way more material taken out of the fore-end mass - and a recoil spring replacement would also almost certainly be necessary. On other hand, I like the way the the 1907 351 WSL fore-end is engineered (with that cool big barrel nut) and I like the 1907 pistol grip. So if I do one for myself, it will probably be the 1907.
The conversion would also make the balance of a 1907 right, whereas, if you have ever handled one of these, you know they are ridiculously fore-end heavy.
The easiest feeding system to set up would be the factory mag - of course. Second easiest would be a Zastava M57 (or TT33) mag, with the mag lips and follower re-profiled for vertical feed. HOWEVER, It looks like opening the mag well to fit a PPS-43 stick mag would work - but one would been to set-up a new mag latch setup. Visually the effect would be awesome - maybe combined with an 18.6" barrel adjustment. That would be "coolski".
