I can't comprehend the spacer you had to use to make the lock work. If the gun worked before, something is wrong as to why it will not work now without the spacer.
As to the for-end did it fit before without a lot of pressure? The only thing I can tell you is to take the wood from the metal and try putting it on the barrels this way you will see what it stopping it from going on.
I have had a few where the "J" spring was very tight on the for-end loop and had to put the "J" spring in vise and slightly bend it to make it fit more easily. I have also had to bend them straighter to make it fit tighter on the loop. This happens mostly on hammerless guns where someone has tried to put the for-end on without cocking the gun first. Putting the for-end on this way puts lot of strain on the "J" spring and will bend it.