When I purchased my 2003 35 Open w/8.1s (recently sold), I had the dealership change the cam shaft mounted pumps to the pulley driven pumps using the conversion kits, as the old cam shaft mounted pumps weeped.
I'm fairly certain that all of the parts for the conversion kit are available individually as the 2005+ 8.1s had the pulley driven raw water pumps standard. The conversion kit just gave you the parts to change the 2001-2004 8.1 raw water pumps to the 2005+ raw water pumps.
These are the part numbers from the parts manual, but you should verify this first.
RA057035 Raw water pump assembly
R065056 Pulley, raw water pump
93940000 Lockwasher, bracket attaching
98443030 Bolt, raw water pump mounting
92934000 Washer, pulley mounting
93934000 Lockwasher, pulley mounting
99335016 Bolt, pulley pulley attaching
R066024A Belt, accessory drive
18402 Hose, raw water pump to transmission cooler