Well, you basically just need the Aeromotive Phantom setup, a new tank, some AN fittings, regulator, and whatever type of fuel line you're into. Pretty easy, but not cheap. I have this in my Duster now. Works well!
You hit the operative phrase: "Not cheap." Usually a factor in Diplomat builds ;)
Even though my Satellite is a rough 4-door, I'd rather it and my Valiant be known as the "Money Pit" than a Diplomat.
There's a nice '87 Dippy on Craigslist right now, zero rust, running, $1k - but do I really want to bother with it if the price turns to $3k by the time I'm happy with it? Not really, given what I already have in the driveway.
I'll freely admit that the Diplomat is probably more confidence inspiring behind the wheel than either the A-body or B-body though. As tight and clean as the Valiant is, there's something unnerving about it - feels as if you're driving a 40-mph-capable skateboard. While I haven't really had the B-body out anywhere but out of the driveway and back, it has that same feeling. Probably not enough interior sound insulation.