Just throwing this out to see what type of response I get. I've been pondering building a 35 caliber b-rod but every time I'm about to start sourcing parts I run into a brick wall, meaning there are no mags available!
I agree about the magazine well being small for slugs but I think Chris just wants to shoot pellets. The pellet mags clear cover can be machined for clearance also. AirgunBuy.com is the manufacturer of that side lever breech, some pretty good pics of it on eBay. George is the guys name and he said he could make a probe and thimble in .357 for his .30 cal breech for me. Chris wants magazine capability before moving forward with the build. It will have the short RAI tube and drop block, JSAR externally adjustable regulator and a 580 cc CF bottle. Probably un shrouded barrel threaded 1/2x20 for a ldc. Use of a thimble will save on some machining.
I have one of hes side lever breeches on my .30 marauder the breech can only accommodate 177/22 marauder magazine thickness.