w_l,
I was in your shoes about a month ago. I read all the posts and finally bought an ASA upper and mag block. It drops in from the top, so it won't ever fall out of my RRA lower. It is a snug fit that is as perfect as it can get. NO play. It accepts colt and uzi mags (and the curved madsen ones you talk about). I modified a few original uzi mags that I got from Cole Distributing for $9.95 a piece. The whole package is awesome. It works great with FMJs. I think I will send my setup to JoeBlack to finish fine tuning it to run with anything I feed it.
I highly recommend an ASA upper and block. I love mine.
Sorry if I sound like a poorly paid "satisfied customer" on a commercial, but it really is nice.
edited to add: All the systems are blowback (I think), none are gas operated, hence the heavier bolt and buffer.
J