Hello. Looks like the problem was only partially resolved. It looks like instances of the carts that are spawned with the players or by command line after installing the mod are successfully changed, but those that are found in the world are not changed.
That is, in a world where the mod is shown to work (Ex. I can spawn a Hydrocraft.HCHanddolly by command line and it would give the appropriate amount of Weight Reduction (For hand cart I have changed that to be 80%, not 90% as above but that should not make a difference to my question,)) a handcart found in a warehouse which is also Hydrocraft.HCHanddolly (Checked using admin mode, edit item) will have only 50% weight reduction.