Your moving away from your original brief. If its ok with your current set up then save up and do the engine but you wont buy bits off the shelf cheaply and if your after a significant increase then your looking a a full rebuild to a higher spec. Hope youve got deep pockets! Id suggest unless you bottom end is tip top then new heads and lare chokes equals a dead engine!
With bigger valves, and wider venturis in the carbs, all your power will move further up the rev range, which you don't necessarily want on a van. You want bottom-end power/torque, so best to strangle the engine so you get faster gas flow at low revs.
