I used the Firestone Destination A/T light truck tires on my Syncro.  The P225/75R15 have a 102S weight rating and they are quite reasonably priced for LT tires.
This size worked well on the Syncro, but I can't say how they might work on a 2WD van.
Ant
      From: Loren Busch <starwagen@GMAIL.COM>
 To: vanagon@GERRY.VANAGON.COM 
 Sent: Wednesday, July 15, 2015 11:53 PM
 Subject: Crazy Idea On Mismatched Tires
   
Okay, this wacky idea popped into my head while driving today.
I now have a 2.3L WBX with a lot more power in my '90 Westy with
automatic.  I could (but cannot possibly afford) go to a higher 3rd gear in
the auto tranny, adds say it would drop the RPM at cruising speed by 800
RPM.  And thus save some gas on long highway runs.
But what if I did this?  Go to larger tires on the rear?  Why only the
rear?  To keep the speedometer and odometer accurate.
Right now I am running 205/65 R 15 all around.  If I were to go to a tire
with larger circumference on the rear, although loosing a little power at
the ground I should drop my RPMs at cruise.
So, is there a fault in my logic?  And would my gain be so small as to not
be worth the effort?  BTW, I haven't tried to do the math on this one yet,
just thought I'd throw it out.