Hi George,
I found
this link, it contains some pictures of models and a drawing of an articulated Ikarus. There's some text too, but I can't read it (I think it's Hungarian), it could give some info about the models you're looking for.
Anyway, since the info seems very hard to find, can't you just make up some reasonable values and release them? You might change them later, when (if) you can collect that piece of information... otherwise your work would stop!
Also: I think I found a truck model which can fill the gap between the Opel and the Navistar

It's the Lancia Esatau 864. The model's design started in 1940, but the first units weren't produced until 1947 because of the war. I think this would be an excellent choice for your 1945 slot. It was a somewhat large truck, built in three variants (a two-axle one, a three-axle one and a semi); it featured an engine of 122 HP and was produced until 1957.
I found some technical drawings and pictures of it, carrying various types of goods. As you can see, trailer versions were quite popular; it was actually a heavy truck and could be very helpful in the game, carrying more stuff than the Opel Blitz as the industries grow.
In the zips, you'll find the technical drawings for the 2-axle and the tractor version (the 3-axle can be made up from the 2-axle); also, there is apage written in English with the main specs and dimensions (it uses Imperial measures). If you need any additional info, don't hesitate to ask.
If you decide to draw this, a good idea would be to slip the Steyr to the 1949 slot, modeling the 380 diesel version, for which you already have the pics (it would be a smaller and cheaper alternative).