I did mine on my 4.3GL a few months ago. I removed the back seat to make it easier to work on; I went with a factory Volvo impeller in hopes it would last the longest; the original was probably from 2004 and was torn. I also replaced the o-ring as mentioned. This was an easy job if you have any mechanical skill and you remove the back seat. I did have to use the "zip-tie" method to compress the new impeller enough to work it into the housing. Be sure to use plenty of glycerine or dish soap to lube it up. After I replaced it, I ran it on the trailer with the back seat out and inspected...the area around the where the gasket "sweat" a little water but not significant...after the motor warmed up this went away and then I checked it again the next time out at the lake and it must have "cured" because no more moisture around the sealing point. That was my experience...let us know if you get it done.
And yes, the raw water pump is driven off the crank (front of the motor) on the 4.3 GL. Does your boat have the water inlet for a hose inside the engine bay? If so, be sure to always use that instead of muffs if/when you run it on the trailer.