Just one legal issue to remember... if you construct a new magazine, then it is illegal under federal law. If you modify it so it works in a Mini-14, and it still functions in an M-16 as originally designed, then it should be ok. There is an interesting article in the Shotgun News, Feb 3, 2003 (V57 I3) which talks specifically about magazine conversion and the BATF's opinion on the issue (page 30). Before you make any mod's, you might want to check it out.
-davey
-davey