How could a brigade that badly organized & led get on US soil? The last time anyone marched out in the open without total air superiority was in Kuwait, and look what happened to them!
Not that this would ever happen in real life anywhere, let alone in the USA as you described it. But if it did and the enemy forces had no HMGs, MMGs, armored recon vehicles and artillery support.
You only need to delay 12 hours? (24 hours minus the 12-hours march time)
We can do this real simple.
DONT BLOW THE BRIDGE! Save this as last-ditch because you will need it for when the USArmy regulars and heavy equiptment come later to repel the beach head. I'm assuming you want to save the town for logistic reasons.
If they're really going to be marching thru enemy country in column packing small arms, undispersed, with no scouts and not having "confiscated" a few dozen trucks:
1) Use the construction equiptment to dig defenses on top of hills on both sides of the road to shoot them down in crossfire & defilade with your scoped riflemen & up to 1/2 of your battle-rifle equiped men. Think trench warfare against an inferior enemy, use your range advantage to pick them off from 1000-500 yards, they'll probably never even get close.
2) Have fall-back positions dug-in on the reverse-slope of these same hills just in case they manage to get past your first line on the hilltops. More assault rifles, lever guns and some shotgunners here. This will cover your possible retreat as well as devastate any attempts to overrun your positions (I'll refrain from explaining the benefits of "plunging fire")
3) Knock down the trees & use the woods as your rallying point for your forces in #1 & #2 above. Use the woods to slow down & harry remaining enemy forces. Dig defenses in two lines, one just at edge of woodline and second 100 yards inside the woods, use your shotgunners & lever actions here since they're useful for defense & short range action.
4) Use airplanes to scout enemy activity.
5) Use unarmed vets as work crew, they'll listen to directions and are used to working togther. Send John Rambo to the front with his knife & bow.
6) Use handgunners as town security force to control looting, communications, traffic, crowd control etc. They're useless in real combat.
7) During the fighting, dig additional defenses in front of the bridge for a "worst case" last ditch effort.
8) Sit back and let the advance Air Cav blow them up.
9) Your biggest problem will be in controlling the Americans. Use a heavy hand to make sure everyone follows orders and stays at his/her post.
Your scoped riflemen would probably kill between 15-30% of the enemy before they routed. Against a determined enemy trained for close assault, your first line would probably kill even more of them, but that's why you should defend in depth. Your first wo lines on the hills should win your "battle".
They'll never reach the bridge in time before Army regulars arrived.