I'd be inclined to lose the 403 and go straight catalyzed epoxy with the graphite. If you can return the 403 and get some 404 even better. And unless you got the 207 for free, return it and get the regular slow hardener. 207 is clear and pricey.
Microfibers act as wicks that will allow moisture to pass along the fibers. Yeah you get strength from them but if your track is captive in the stock it's not really an issue unless you have a long, thin stock.
Using 404 and graphite, you'll want to go 25% / 75%. The 404 will bulk up the resin and still let the grahite do it's thing and as it's lighter and finer it will float to the surface of the resin while it cures. And here's the thing- you gotta really watch the heat buildup. If you're in an area where it's going to be 70F plus, toss the resin in the fridge for a half hour first. This will chill it and thicken it as well so you'll also have to be real aware of your pour, working the completed our to get the trapped air out with a few good raps against the bench and a consatnt vigil of offgassing.
Using moly would be OK if you could be sure that the stuff was encapsulated and so wouldn't contact the salt water. Since you're going to mill the track after it cures then all bets are off. I'd try to off the moly online and go find some graphite, perhaps the retailer where you got the 105/207 from has the graphite filler and will take the return of the 207.