No, it's not practical. Thanks to ATF import regs before 3/89 you need to do a heck of a lot more than some silly "kit" and an "autosear" on the HKs. Like Gary sez, if you are a Class II FFL or the government, go see a T&D guy.
No, it's not legal. However, you're making a new emmagee after 5/19/86 whether from a block of metal or an existing Title I firearm. What Buzz said. Another exception, Buzz, you have a pre-5/86 Form, aka "paper receiver."
Get an SOT and start knocking on doors of PDs.