Check this out - I think I'll be OK.
This article discusses some Marlin lever action conversions. The author was actually able to stuff a .50 AE round into a Marlin - WOW!
What is Possible with the Marlin?
The big issue seems to be the behavior of a thin-walled chamber that causes bulging - due to the fact that the threads are inherently weaker than other designs. That will be eliminated because I'll lathe off the threads to prepare for a sleeve of 4130 steel. For a 1" AK trunion, that will mean I'll have about .4" around the radius of the chamber.
That thick a collar of steel in itself should have enough mass to eliminate bulged/burst chambers. It would certainly take 50,000 CUP and since it appears the barrel itself is made of a high-tensile steel, I should be OK even with those 500 grain slugs.
The only question that remains is whether or not I need to obtain a throating reamer to open it up for those larger slugs.