I have a D722 & a D640 in the bed of my truck in the garage. If I measure the center distance from the crank to water pump here is what I get.

The D722(out of a Grasshopper ZTR) is about 8.25" and the D640 (out of a 1772) is a little over 7".

It looks like the gear case on the front of the engines are different making the center distance of the water pump different. Let me do some more checking on my other Kubota's to see what is going on.

Here is a pic of the D722, not the correct fan blade for a Cub Cadet. You can see the back of the water pump in the second pic.
Attachment 89347 Attachment 89348

D640 -- look how low the water pump is.
Attachment 89349

Cub Cadet 2182 engine
 

D722 crankcase is taller than D600 crankcase because of longer stroke, therefore the gear case covers and water pumps are different. Engine side mount bolt holes have same footprint. Crankshafts are different, crankshaft pulleys and flywheels are the same. Connecting rods are the same length, large end is the same(use same rod bearing) but the wrist pin on a D 722 is larger diameter. Motor mount holes to centerline of crankshaft or the same. I have tons of experience on D600 and D722's, no experience on the gasoline models.

Good to know Greaseburner. I was thinking they were pretty much the same but didn't think about the stroke.

I can't figure out why he is having clearance issues with the fan. I'll see if I can snap some pics of the engine on my 2182.

Read post #12 and 23.
There is a different distance between the center line of the crank shaft and the center line of the water pump between the WG 600 and WG 600 B.