One of the best $20 you'll spend is on a 0 to 1 Volt Lascar LCD volt meter.

http://my.tbaytel.net/guskers/conversion.html (gauges section)

This, assuming your O2 sensor is working properly, will give you a very
good idea of what is going on with the fuel system.  If you see the needle
flicking back and forth around .5 volts, dropping to zero on decel's and
jumping to .8ish on full throttle, all's OK.  That will give a very good
indication that you are operating at peak (which isn't saying much)
efficiency.

As already mentioned, speed increases do not behave in a linear fashion
correlated with wind resistance.  So keeping speed down, on a properly
inflated set of tires, will be most effective in improving mileage.

At current, and increasing, fuel costs, a conversion becomes justifiable
fairly quickly if you plan on keeping the van.  I'm seeing about 23mpg
(US) / 27.5mpg (Imp.) highway with the I4 conversion.  At the current
$.95/litre there's a fuel saving of $4100 dollars over 100 000 miles with a
5.5mpg average differential.  If gas rises to $1.10/litre it becomes
$4700.  And that's with 135HP.