1-5 duneable override is the way to go.
If you only, ONLY run 300 foot sand, 2-5 is best, but....if you decide to go to pavement, or the dunes and run a 400, 500 or longer race...you'll need 5 gears....
I run 300ft 98% of the time, but...when I go to the ice, it sure is nice to have 1st gear.
1-4 will work, but for launching hard in 1st gear, it is tough due to how low the ratio is.
I can get you the best deal on the net on override tranny's...
I've had a 1-5 N down dunable in mine for 2 years, flawless....