I pretty much agree with most of what Absinth says above, mainly bar the 2nd last paragraph. I've always been an advocate (even back in the mid 80's), that if you were going to build a modified car, start with something with intrinsic value. So as an example, if you wanted to build a hot Torana, start with an SLR5000 and go for broke from there. In the early 90's an rolling SLR5000 with very little rust was worth $1000, a whole car in good nick $3000. Stuff around with it and it would always still have intrinsic value. If you took the slightly cheaper way out and bought a Sunbird or 4cyl Torana you ended up with the same thing, but it had no inherent value other than the parts you put into it.
Come forward to today and you want a nice, good looking van or ute. To me the smart thing to do is find a Sandman. Spend $15,000 well on a Sandman and you'll pretty much not lose your investment. Do the same on a WB van or an original 6cyl 3spd HX and you throw $10,000 away. So to use Absinth's example of an L31 M21 HQ Sandman that has had a WB Statesman front fitted, HZ GTS dash and Statesman interior fitted, 9" diff fitted and painted Hothouse green, if done right to me that is still at least a $12-$15,000 ute (or van) (maybe a HQ isn't such a good example though as it'd have had structural mods to the dash and cowl, so lets say the same thing but in a HJ!). Do the same to a base model HJ-HX and to me the van is worth the parts that create it.
I'm actually using the same philosphy with one of my HK's. The Warwick yellow GTS327 I have will one day cop a 427, Muncie, 12 bolt, Harrop brakes and 18x8" CV8 rims. Probably have a cage and harness as well. I have a near perfect HK 6cyl GTS shell (Silver mink with red trim) that would be a lot easier, but to me the base value isn't there. The GTS327 has a tag and chassis number inherent value of say $15k, the GTS probably $1,000.
Lastly, on body swaps, I have no problem with it but not for all examples. Some have firewall numbers and other features you cannot replicate without effectively breaching rebirthing laws. Some bodies you can replace with an identical example with no identifying numbers, but the body must be from the same assembly plant, series and month or it isn't worth doing in my opinion.