Wrong forum, this should be in the worldbuilder questions thread.
To answer your questions:
You can find Howitzers under: ebps/races/allies/vehicles/m2a1_105mm_howitzer
and 88's under: ebps/races/axis/vehicles/88mm_flak_36_at
But you should destroy them via the "edit state" function, otherwise they could be recrewed and that would give an advantage to the faction that captures them.
Other wrecks can be found here: ebps/environment/art_ambient/objects/vehicles/military/wrecked_vehicles.
A problem with wrecks is that they give bonus xp when they are completely destroyed. As mentioned in this thread:
http://forums.europeinruins.com/index.php/topic,5356.0.htmlIf you got the latest patch for CoH (ver. 2.301) your worldbuilder should be up to date as well.