It looks like the AMS unit would drop right in like you say. And as you implied a SPAL fan is thinner and from all my experience they move THE most air of any electric fan...They do not have a shrowd though that would help draw air through the entire radiator.
Does the AMS verbage say that the evaporator must be removed? I don't see why it woul unless you do plan to run your turbo and header combo through the new space left open by the smaller radiator. My evaporator would be fine where it is no matter what size radiator I used.
BTW - Glad to see you making progress on the EVO man! Maybe this time we will have our two out at the same time.