This gun has a round action with no flat or lug to grab the action by. The mag box/trigger housing is welded to the action and is sheet metal ... not a good place to grab. I tried a piece of wood in the ejection port and merely succeeded in bending the lower rail. Any ideas?

Trying to remember. Didn't those have a barrel collar and pin? Been way too many years since I've held one. Assuming the collar and pin aren't holding it in place it would be an action wrench pretty much like for a 700 round action.

I made a set of wooden pinch blocks to hold the action in my vise and then used the barrel vise on the barrel with a mallet. The action is perfectly round with no facets. 700 style wrench does not work as there are no lug raceways .. action is round inside as well. Two pipes threaded together ... pipe wrenches are designed to solve this problem but that would chew things up.

You could epoxy some pieces of metal to grap
the pipe wrench. Somthing like a rod bearing.
Heat will remove the epoxy.

Unfamiliar with the Marlin but have seen some bolt action shotguns that were all 1 piece ie the receiver was machined as part of the barrel. You sure this gun is a 2 piece assembly?
Just a thought.
C.G.B.

just epoxy a block onto it... tap it off later, torch will take the residue right off.