* Front of the cage:
The first cut you need to do is at the front of the cage to allow the Grenade Launcher Front Block to slot into it. (In all the pics in this pictoral guideline, the first pic is the unaltered cage, the second pic is the rough guidline as to where to cut, the third pic is the material that would be removed, and the final pic is the altered cage.)

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Step 2: You will need to secure the receiver in a padded vice. Lubricate the inside surface of the aluminum receiver block. The tolerances are very tight here. Using a brass or blunted steel pipe and a soft mallet, tap the block onto the Thompson barrel orifice until flush. Take care not to damage the airsoft barrel. Use the adjustable wrench to field adjust the block so that it is aligned perfectly with the receiver.

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* Left side of the cage:
On the left side of the cage you need to remove three small sections to allow the pump grip's racking mechanism access to the inside of the cage.

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* Bottom side of the cage:
To make the cuts for the loading port, you need to extend the length of the hole that's already there like so:

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Step 5: Insert the steel barrel sleeve over the brass airsoft barrel and into the aluminum receiver block. It should slide in between 3" to 3.25" depending on your preferences of barrel overhang. Test fit everything to see what I mean.

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* Right side of the cage:
IMHO, the hardest cuts to make are those of the ejection port. I start by cutting the small section at the top, followed by the long horizontal cut at the bottom. Two vertical cuts completes the rectangular cut out. All that's left is the curved cuts:

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Here's a close up of what it should look like:

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Here's a little extra info. Personally, with the way my Pulse Rifles are constructed I don't bother with the following two cuts because I'm using a resin M870 receiver, and I attach the triggers seperately, so I can simply slot the receiver through the front of the SPAS12 cage (and not worry about the trigger getting in the way). However, if someone plans to use either an airsoft or real steel M870 receiver, they'll have to load the M870 through the back of the cage and therefore needs to do the following:

* Back end, Major cut:
To be able to load the M870 receiver through the back of the cage, you'll have to remove the following material:

Actually, I've probably highlighted more than actually needs to be removed from there, but you get the idea. The cut can probably be more squared off, like on the altered cage on the right. I found this cut to be about the same difficulty as that of the front section cut... which was very easy.