So I'll explain it as best I can.
As mentioned above, the T3's cooling system capacity is enormous. Not because of some extra need to cool things, although the fact that these are technically commercial trucks in a sense does call for that... but the real reason is just the distance the radiator is from the engine. They hold as much coolant as many big V8 engines do. Heck, the big 4.9L I6 in my F150 holds less coolant.
So, Volkswagen added the extra, non-pressurized overflow bottle. It would have lived right inside the engine bay, and its level and fill cap can be viewed and accessed from the drop down flap where the license plate goes. The bottle they used, which is the expansion tank, was mounted right next to it, and was the same oval-shaped arrangement common to other VAG products of the era, like what an A2 Golf or Jetta or B2 Quantum or something would have used. BUT... the cap was a special T3-only setup, with a little nipple made on to it, and a hose that went over to that extra tank.
In essence, they piggybacked two types of cooling systems into one. The modern, pressurized, setup with the expansion tank just like every other Volkswagen in the modern era uses, but also the old-fashioned vent/recovery tank like some older cars as well as some newer cars still use today. The volume in the expansion tank doesn't allow enough room (the area above the centerline... sometimes marked MIN-MAX) just isn't large enough to accommodate the amount of expansion necessary for that amount of coolant.
The newer tanks that an ALH car would use do not allow for that type of cap to be installed. So you've got to deal with some way to try and alleviate this discrepancy if at all possible.
1983-1991 VW Vanagon Cap For Expansion Tank - BLAU Germany - 025121321-BL (jbugs.com)
If you are still using the old style tank, that allows the use of the small cap, then all you need to do is plumb in some overflow tank somewhere, that maybe can hold a couple liters of liquid. Bad side of doing it like that, though, is that HOAT coolants (like G12) do not like that exposure to atmosphere (T3s all left the factory with the blue G11 type coolant, which isn't as bad about the additives dropping out of suspension like G12 is).
So another thing could be to simply use TWO modern type tanks. That would double the expansion potential... which would help, but it still wouldn't allow for that nearly thrice the amount of coolant capacity you are dealing with. But would certainly get you closer.
And yes, GoWesty has that nice aluminum tank that would not only help but also uses a more old-school conventional "parts store" type cap, with the nipple to the overflow tank made on to the pressure tank (like you'd normally see) instead of the hastily modified and failure prone cap that Volkswagen put on them from the factory.