Not all GAZes 53 seen in Bulgarian movies have been renamed, but only few (well 5, like has been stated) - late ones, as not all BG-assembled GAZes were considered as the “400-series”.

This forum http://scalehobby.org/index.php?topic=1722.0 says, that later Bulgarian assembled GAZes with Perkins engine were called for example:
- 401
- 402 (turbo)
- 411 (tipper).

More websites/forums, which mention the Madara 400 naming:
http://en.wikipedia.org/wiki/GAZ-53
https://www.facebook.com/media/set/?set=a.167435356650388.39158.166588963401694&type=- 1
http://www.33gruzovika.ru/encyclopedia/m/254.html
http://www.mglnews.mn/content/38157.htm
It seems on one website, which I cannot find now, they quoted as source of similar informations Czech book about the Skoda/Liaz trucks.

And here is the Polish annual catalogue "Trucks of The World", edition 1998, which I own:


Page 110 and 111 – short info about the Madara activity. The underlined words say:

Till 1992 following models were assembled (by Madara):
…
- 401/402 = GAZ 53A, 6-8 t, Perkins engine.

So it might deserve further research, but if sources from various parts of the world mention the “Madara 400” naming (even with such details like submodels), it can't be ignored.