as these vans get older they require a little/lot more maintenance, while i was in changing the seals for the intake plenum and i had the top section of the motor apart i went and changed my plugs as well and while you have a missfire on 2 cylinders you might want to check the coil pack located under the cowl attached to the valve cover by 3 bolts. when i pulled mine i had spider cracks all through the epoxy which didn't cause any mis firing at that point but i replaced the coil as well
so when i did mine i did:
6 spark plugs
sparkplug wires
coil pack
isolator bolts
all the gaskets
front valve cover
pcv valve
fuel regulator vacuum line
most of it was replaced as when you buy the valve cover repair kit it comes with everything including the new valve cover as the factory part allowed too much oil into the intake manifold (Poor Design) kit is put out by dorman and the number is 615-177 :
https://www.dormanproducts.com/gsear...Repair%2520Kit
soo all you need to do is buy the plugs, wires,coil pack and your good to go took me all of about 2.5 hrs to complete but i was cleaning parts and taking too many coffee breaks, if your going to keep it for awhile i would also suggest changing the fuel filter if you don't know when it was changed
there is also a stick about the p171,p174 issue on the forum i found it useful as well