first off, the 450 foreman is a solid rear axle, so the ride quality will not be NEAR what you had on your 660 grizzly
so expect a LOT more bounce to things, and its NOT made for two up riding either??
as for waterproofing ANY atv
its all about sealing things,and protecting electrical connections and raising breather
vents
SO< that means dielectric grease on ALL electrical connections
next will be raising all vent lines, from crank case to diff's, then the snorkel and making SURE the air box is sealed RIGHT
failure to do all this, and water can get into things it shouldn't and either stall bike, or worse
a heads up, a very common issue on the 450's is left rear axle wheel bearings.seals
even a LITTLE wear and water will get in there and RUIN the whole rear axle
good news is, its a easy repair and a cheap one, bearings and new seals are like 30 bucks, and a little time with basic tools
left side has ONLY one bearing, right side has two, hence the weak link on the left side
a rear axle is about a 175+ bucks, so its worth changing them as needed rather than waiting
450's are also very reliable ,long living atv;s
so congrats on it, but its NO smooth riding atv by most standards!, its a WORK atv more than aplay /trail atv!