Are you worried about the speed with a 1/4" port? Because the Datsun goes up faster than my truck, which is bag over bar.
Anyways, I know that Firestone makes a larger sleeve bag, that is designed to carry a load. Its a little pricier, but its probably what you need. Can be found here:
http://www.ridetech.com/products/Airspring_lb_1T_rolling_sleeve-985-0.html