Mike:
I think most guys who try to replace one of the MK I barrels make two mistakes:
First they hold the receiver in a vise, not the barrel, and try to unscrew the barrel.
The other mistake is they get the barrel in a vise and then figure to use a piece of square steel stock in the recess for the bolt "wings" on the very end of the receiver!
The first one collapses the reciever, the second twists the snot out of it!
Correct procedure: barrel in a good vise, designed to hold a tapered barrel, then use an action type wrench that is flat on the bottom, to match the area that sits in the pistol grip, and round on top to match the rest of the receiver contour. Then get it as close to the feed ramp end of the receiver as possible, without hitting the feed ramp when you turn the receiver, and apply pressure quick and firm to "pop" it loose!
As far as the insides go I don't see what you would get ahold of or be afraid of bending as almost everything comes off with either the bolt the grip frasme of unscrews with the barrel.
Of course we now come to the lazy mans way to remove a MK I barrel!
Put the barreled receiver in the lathe and cut the "ring" that butts up against the receiver as a stop. Once that is removed I suspect the old barrel will unscrew easily because it no longer has any torque on the threads! The down side is the old barel is now scrap!