Think a .450 #2 might be a better solution. The larger the case, the lower the pressure for a shotgun conversion. .450 #2's have been done and posted here at www.nitroexpress.com in the past due to the lower pressure - something in the vein of 36,000CUP or 41,000PSI. The .450 Nitro is close to that at 44,000PSI and 39,000CUP. I'd feel much safer with those pressures in a shotgun frame, over the Lott's 54,000CUP, which is also listed as 61,000PSI.
http://kwk.us/pressures.html

A .500/465 might even be better, but would probably need a 12 bore frame?