Well, here's my thoughts.
First, you need to get those planes in the air, flying in relays to monitor the enemy's progress from beyond small arms range. Just because they were on the highway doesn't mean they'll stay on it. Keep tabs on them, and establish communications with the Army forces coming in so they'll be up to date.
Second, arm those 500 vets. Ask all the folks who brought guns to the party if they have spares they can loan to a fellow defender. Also tap the local gun stores, as was pointed out. You want battle rifles first, then lever actions, then shotguns.
Get whatever load-bearing equipment you can. Every man should have a backpack with ammo, food and water. Also find some flares and floodlights with generators in case of night action.
When you start dividing up your troops into units, give them catchy unit names. It's a morale thing, like Patrick Swayze yelling "Wolverines!" For leaders, pass over the barstool commandos and choose the calm and competent types.
Get your folks with the boats to remove all boats from the far side of the river, working outward from the bridge. Don't give the enemy any means to cross. Also evacuate any civilians on that side. Tell them to bring their guns.
Get that bridge wired with enough TNT to blow the center span, but hold off on detonating.
The folks with handguns won't be very effective at the ranges we'll be fighting, so they get to organize the evacuation of the town, and patrol for looters. Split them into 3 man patrols with assigned territories and give them radios.
The battle plan is to use multiple lines of resistence, but not to fight a pitched battle except at the last resort. The first line is at the far side of the bridge. Get your scoped rifles on high ground; their targets are officers and NCOs plus anyone carrying a radio or RPG. Reinforce them with your auto-rifle guys. They will set up on both sides, aiming down the road with overlapping fields of fire. Use cover and concealment. When the invaders approach, wait till they're well within range of your weapons -- about 200 yards, maybe. Then your guys have 60 seconds to dump as much fire as they can into the bad guys, then dash for the river, where your boats will take them across.
Once across the river, they will be dug in along the bank (entrenchments provided by that construction guy) and interdict a crossing. If the bad guys try to cross the bridge, then you blow it, with them on it.
Behind the guys at the riverbank, there are more trenches, with at least 100 yards of clear fields of fire. Bulldoze trees if you have to. Your lever-action and shotgun troops are already there. If the enemy does get across the river, this is your fallback position for the guys at the river. If you have time and materials, you can put obstacles out in the, like concertina wire or just twisted chain-link fence.
Behind this set of trench lines you will have pickup trucks loaded with ammo supplies and more water (water is important). Put another set of trenches every 200 yards back to the edge of town. At the bottom of every line of trenches, bury sticks of dynamite with 55 gallon drums of gasoline on top of them. If you have to fall back, you'll blow these when the enemy reaches the trenches you just left.
If you have to fall back into the town, have prepared high positions for the snipers, with stockpiles of ammo and a couple spotters each. Spotters will have small arms to protect the sniper, and binoculars. Block streets with cars and trucks. Your tactics here will be to use your local knowledge for run-and-gun -- you pop up, hit 'em quick, and vanish to pop up again elsewhere.
Prepare a facility for prisoners and one for medical treatment. Have fire and rescue teams standing by.
Then get out there and kick some Moronian butt.
"As I looked at my two young sons, each with his gun, and considered how much the safety of the party depended on these little fellows, I felt grateful to you, dear husband, for having acquainted them in childhood with the use of firearms."
-- Elisabeth Robinson, in The Swiss Family Robinson by Johann Wyss