Kimmo, you said the white bits were made by you from styrene, except in the photo it all looks white - or is that just me? I think I have seen that kind of dodgy quality moulding before on a Zvezda kit. Lovely unusual subject though.
Tim, that abandonment of the never seen by me Airfix Vickers tank is a bit disappointing. Are you going to just consign the whole thing the spares box? I think this is the tank with the Carden-Lloyd type suspension? Perhaps you can be enticed to knock up an old-style conversion with it to make something completely different?
On that subject, I have been poring over some almost 40 year old Airfix and Military Modelling magazines, and have come up with something for Jan. I must confess to being slightly confused, are you combining these two kits into one, or are you finishing both of them off?
Anyway, from Airfix Magazine, October 1971, here is some of a Terry Gander article on making an SU76i from the Airfix Stug III kit.

I had to include that wonderfully presented template... how things have changed . . , I found this while looking for the Opel Maultier rocket launcher build (based on two Airfix Bren Carriers) and the Matilda workshop crane, both of which I bought kits for. I must say, the Matilda looks a lot easier than the Maultier.