Need gear ratio , tire and rear end ratio experts ! Green Rat Project

Check your first gear ratio, as some of those NV3500s are reported to have had 3.45 gear ratios. 1st gear is going to be great for putting around at parade speed, but very traction limited. But are you prepared to make room for the NV? It's probably going to take some work to the torsion bar crossmember and definitely a redesign of the transmission tunnel to fit. And here's the thing about it. A stock 5.2 magnum with a mild cam (360 LA Hydraulic roller works very well, BTDT!) an airgap type intake, and a decent distributor curve makes earthmover bottom end torque. It works well with 3.21 gears and 31X10.50s in a loaded 4X4. So I would go with 3.23 and keep the 3.91s held back for when you go "all out". So by my own experience, you'd be highway only with 5th gear, but it would pull 2900 pounds very easily at highway speed. But, here's a helpful online calculator to help you determine what will work for you, click the button for MPH from KPH to go to standard tire measurement.
Gearing Calculator | RPM and Tire Diameter to km/h - mph with Shift Tracer