I did the opposite actually. I put matrix shrouds on an HCG foundation (resin thompson and GL). I had to shorten height on the Matrix shrouds because the they are taller. I believe they did that because the snow wolf/Matrix GL sits lower and needs the taller shrouds to account for the gap.
I would imagine you would need to figure out how to raise the GL closer to the thompson barrel if you wanted to use the aluminum HCG shrouds. You would also lose locking function of the stock because the locking mechanism is build into a bulky and hollow area on the matrix shrouds. The HCG wil be solid metal and lower profile. You will def need to put some work to make it work, but I am sure it can be done.
Again, all these pics are "reverse" of your plan, but they do illustrate how the Matrix is overall Taller than the HCG and you will need to account for the height difference in fit-up.
See the gap here (shrouds build for a lower set GL)
Attachment:
20150608_191125.jpg [ 161.96 KiB | Viewed 1972 times ]
To fix that, I just deleted the excess material in the counter area to bring the shrouds 'up'. In your case, you'd have to move the grenade launcher up to fit the HCG.
Attachment:
20150608_191041.jpg [ 334.78 KiB | Viewed 1972 times ]