>I'd ask. RLL drives use ST506 and MFM use ST412, or am I backwards?

I've forgotten the rule on this exactly, but you can use a RLL on a MFM ST506 interface, but at a greatly deminished capacity (50%).

RLL is a form of MFM.  The interface is compatable but there are some gotchas you have to look ot for.

1) I think you have to assume there are 1/2 as many sectors per track.
2) You have to re-low-level format the drive.
3) Not all drives will tolerate the change.
4) I'm going on 7 year old memories and can't be trusted :-)

jam