I'm pretty sure that it's a 630V (been a long time since I looked at mine), but if you fold down that little flap of metal after the "HSW" in the model name, it'll be clear.
The Hurth Trans Noise service advisory I linked above suggests simply sucking out what you can, and calling it good when you convert "It is not necessary to flush the transmission, just get as much of the old fluid out as possible."
Delvac and Rotella are both fine oils; I'd buy whichever one is on sale. Both have the automobile rating called for by Indmar, so they're acceptable for use in the engine/trans. In general, the diesel oils are seen as being of higher quality than auto oils. For example, they had higher levels of zinc anti-wear additive for a lot of years, because in a car the zinc additive tends to degrade the catalytic converter over time which isn't a concern in a diesel. I believe both Rotella and Delvac reduced zinc levels a few years ago, so that may not be an advantage any more. They're also used in the heavy use, high power environment of over-the-road truckers where things like engine wear are tracked, so there's more incentive to keep quality high than in the auto market, where sales are driven by how much is spent on TV commercials rather than maintenance records. It's not clear if this is really true, but it makes me feel better so I use the diesel oils.
1998 Maristar 200VRS