Mine's a Dodge Cummins OEM fan. Knowing Cummins they're probably built to some pretty high specification. There's several good companies out there, looking at JEGS gives several offerings, Derale being one noted company. I'd just get the bigest that can fit in the opening, with a good possibility that a single large fan can move more air across more radiator surface than dual fans. 1 @ 18" fan is 254 sq. in. [figuring pi x radius squared] while 2 @ 9" fans is only 127 total sq. in of surface.

It was relatively easy to move radiator forward, but too they're heavy enough that an extra pair of hands can prevent damage to the core when juggling around. Personally I prefer to have it hanging free in brackets rather than to use rubber isolators under bottom tank.