what trans is in that van, if it has a torque converter clutch it may
not be disengaging which will could cause a similar concern, if it does have a torque converter clutch i would get a wiring diagram and find out how power is routed to engage clutch and disconnect it and see if that helps, as for the e-350 with a 460, everything is crammed and tight under the hood of vans, i hate working on them, i believe there is a special tool for adjusting the carb that is pretty long and you will probably have to access it from through the dog house, sorry if these answers don't help much i dont do much drivability work, mostly front end