I have worked in the automotive charging arena for over 25 years and.....
You will never ever hurt a car by installing a larger alternator than came from the factory. Meaning more amperage will hurt nothing. Your van will only pull the current it needs. Lets say you have a total amperage load of 85 amps with everything running. Which alternator would you rather have, the 90amp or 150amp.
You would rather have the 150 amp for reserve current and also less heat. The larger alternator can provide 85amps easier than the smaller one due to size. The 90amp alternator will run hotter trying to produce 85amps.
My 05 has 4 different alternators that fit. They all use the same bolt pattern but the physical case of the larger amp alternators is bigger.
Put the biggest amp on you can afford. Never put smaller than the factory installed.
The only downside to a larger alternator might be a lost horsepower or 2 because of the increased load on the engine turning the larger mass of the alternator.
Charge wires from the alternator to the battery are also important. You can never have too large of a charge wire either, The bigger the better, just make sure it is properly fused. I like to use a 2 guage or larger wire on all cars. There are not many factory charge wires that I consider big enough. They all are normally sufficient but again, the smaller the wire the more heat and heat means resistance in the electrical world.
With the van running measure the voltage output at the alternator and then at the battery. Are they the same number. Ill bet they are not, there is always some loss in the wire from the alternator to the battery. The larger wire will loose less.
My favorite wire to use is welding wire. It is multi-stranded and the casing is designed to handle a lot of heat. Stay away from aluminum wire if you can. Copper is the best.
Sorry to wonder off your original question, but here is another tip.
Diode based battery isolators are crap also. I have measured over 1.5 volts loss through them and they produce a lot of heat and resistance. Replace them with heavy duty contactor switches which are rated for continuos duty not momentary duty such as a starter relay would use.
Here is where you can get the good stuff. I have been using this company for years and they know their stuff.
http://www.wranglerpower.com/