Miles on my van

Yes and no. New engine for one legally needs to be done from the factory, and your odomoter is still going to say the same. Also, every other component besides the motor itself is going to still be old and worn out, for ex: the transmission, motor moutns, bushings, suspension, radiator/belts etc etc etc.

And Spector if you PM or tell me in here what kind of van it is, I can show you pretty easily in about 5-6 steps (provided you provide pics under the hood) of what to do and how to do it.

Hell, with 'Seafoam' a can you can pour about 1/3-1/2 the can in your gas tank alone and that'll do a bit of flushing, and with your motor completely cooled off you can pour it into the oil resivour, (where you put the oil in) and it'll clean the heads pretty decently, also when you run Seafoam you need to run your car for about a day and it's best to change your oil shortly afterwards because of all the gunk it releases off of the cylinder walls/etc.