Maybe they are slightly different weights, but they are all essentially the same flywheel - They all bolt to a K crank, are all 180mm, ring gear is all the same, and all have a +1mm step.
The only difference with the KM20 flywheel is it has a 200mm clutch plate, and hence the mounting bolts for the pressure plate are further out.
You can't use a 200mm KM20 flywheel with a with a K40/50, as there is no clutch kit in that diametre with the right output spline!!
You can only get 200mm plates to fit W50's and T50's.
Right, something else to make note of with the 180mm clutch/flywheels:
There is 2 different splines available in the 180mm clutch, being 21.6mmx19T and 24.1mmx21T.
These are not 4k or 5k etc, but K40/50 or T40/50, obviously the bigger is the T40/50.
Unless you have a f@$ked rear main seal which is spitting oil on the clutch, I reckon it just needs some adjusting. Short of that, how old is the clutch!? Personally I think that PBR replacement clutch kits are better than RPM heavy duty kits!!!!!! I've had way more issue with slippage with the RPM kit, when I used to be able to do 3rd cookers with a 25+year old PBR kit!!! :D