The six should be OK for around town, it makes almost as much torque as the 4.6. Where you really see the difference is if you spend any time on the interstate at speed.

If this van is a vanilla cargo van with the six it is very likely to have the E4OD transmission. That's the tranny that would come with the E250.

You should be fine swapping out the rear end, springs front hubs and brakes.

Steve