Adapt and Excel
Ok.. Imagine the posibilities... A spacer would simply move your existing wheel (or similar) outwardly to the limits of what stud length would allow a lugnut to somehow keep the wheel attached to the truck (And you can buy longer studs). To be able to go further, you would have to bolt the spacer to the truck, then be able to bolt the wheel to the spacer (even those you can buy). An adapter you (we) are thinking of would be similar but now have 8 lugs on the outside instead of 5. (and yes, there are adapters to change bolt pattern diameter but still staying at 5 lugs) Mechanically, this is quite simple to do. BUT (and this is quite a big bubble butt) now the wheel is offset way outwardly from its original position, at least equal to the length of the studs that you originally bolted the wheel onto. That puts alot of stress on wheel bearings, ball joints (which aint all that much to start with) not to mention that the tires will now be chewing on your fenders. Anyway, just because they make spacers, doesnt mean they work for everyone (some people dont mind monster repair bills each time they go 'wheelin).
Your dad (and others) likely had limited sucess with the VW adapters because of several reasons: Reduced vehicle weight; Different bearing/balljoint arrangement (if he even used them on the front axle); and Modified wheelwell clearances.
In a Knutshell, you (we) asked good questions, but I believe the scarcity of 5 to 8 lug adapters on the market is due to the very (VERY) limited real world applications that they would actually be sold to satisify. And that the line was drawn between 5 lug (even 8 lug) spacers and 8/5 lug adapters was due to the inference that a heavier rating would be imparted from the 8 lug wheel/tire to the lighter rated 5 lug axle of the truck. This is my belief, I stand waiting to learn just like you.
My suggestion is to get wheels that work with the tires you have (look at simple steel wheels), then sell the 8 lug wheels. Your math may be different but Im guessing the end result would be about the same as getting adapters (if you find them), and would work A.L.O.T better in the long run.