to do a transplant? (2) Which way to go: which should be the donor?
>     (Yes, I do want the pop-top!)

Well, since you insist... one last question first though. Do you want the 
pop-top AND the bed or is the pop-top alone sufficient? If it's just for the 
roof, then I'd lean towards transplanting the Westy stuff to the Syncro. I 
think you are in for a nightmare of changeover problems converting 
non-Syncro to a Syncro. The Bentley manual shows different underside 
dimensions for the Syncro, and that is likely just the beginning. 

Option #1) For moderate off-roading why not swap a Syncro trans with locking 
differential into a Westy. You'll get a 5 spd with granny gear and great 
rear traction. Hill climbing would be good. This in itself may be a fiddle 
to fit.  

Option #2) As above, without granny gear, get the Lock-Rite limited slip 
diff from Weddle Eng. $300 plus installation into your existing trans. This 
would help greatly. Regular 1st. gear is pretty low to begin with. 

My limited experience off-roading 2WD VW busses over the years has been that 
once you get up to a steady speed, well up in 2nd gear, you can make it over 
amazing stuff, sloppy mud, snow, shale beds, blast 20 feet of deep water. 
Where the locking diff should really help is when you have to get going from 
a standstill. Desert driving requires speed, to get on top of the sand, 2WD 
works fine once you are moving, if your suspension can handle it. Lots of 
Peugeot 404 2WD stationwagens/pickups in North Africa. (Lots of Toyota 4WD 
pickups too!) 
         The granny gear in my Syncro is great for crawling up hills, as is 
fulltime 4wd. But I don't know when I'll ever really tax the beast to its 
limits, like Derek Drew does. What is lacking however is the ability to take 
big bumps at speeds over 20 mph. This is not a Baja buggy, it's a heavy box 
I can sleep in. I used to be able to drive faster in an old Citroen DS21 
with fully adjustable long travel hydropneumatic suspension, a truly amazing 
system. 
        Figure out how far, over what, you want to go, then build what you 
need. I'm still trying to find a wrecked Westy to transplant into the Syncro 
BTW, this being a weekend DIY job (without the roof!). We still take a tent, 
standup outside etc.   tim s.