I'm using 2 of the 3 you asked about. The front oil seal is one of the best mods you can make. It reduces your drips and runs by one!
The other I have installed is the pvc system in place of the road draft tube. It also helps keep the engine clean. I made up my own - bought a simple pcv valve, a section of hose, adapter for the threaded hole on the intake(stock) and a pcv plug that fit the draft tube hole. (was on the same rack with the pcv valves at the parts store)
The one I haven't tried is the full-filtering system. It makes sense that it is the way to go, but you have to remove the engine to install it. Good candidate for when you rebuild. I'm doing what some old timers used to do. I removed the inadequate filtration system and just change my oil often.
- tim