I'm sure we can sell the blocks by themselves. Give me a few days to work out the price and get a page set up on our site. Once I have it figured out I'll post a link.
I see no legal issue with this, but apparently it would be ill advised to transport the empty bodies... and for what it's worth I suggest you dispose of the original base plate retainers after you replace them with the blocks since without the retainer it would be virtually impossible to assemble the magazines as standard capacity "prohibited" devices.