Ciao Lamb,
Interesting ideas and thanks for sharing your very nice build.
a. I agree about the use of a Sig33 from the Panzer II version. These elements I think you mean as indicated below are as per a Sig33 in a Panzer II.
b. In this next photo, I’ve indicated 2 parts missing from your Sig33 (the left arrow shows a change to the Pz II gun version)…
There has been a rear end plate and bracket attached in the position shown below by that stage of the conversion.
You have removed the two extension points at the base.
It is highly unlikely these parts were removed and likely formed attachment points. The front extension internally allowed for sideways movement for aiming anyway, so impossible for that one.
As per the installation photo, there is evidence that the gun was inserted in the tank with the two extensions shown and not removed for several plausible reasons. Basically, in your system, the base of the gun at the front was welded to one girder. This goes against what was determined in the factory for the Panzer II to deal with the gun weight and when fired.
It is extremely unlikely that the girder support for the base and heaviest part of the gun was reduced to one girder in the field. They had a way of supporting the gun determined by engineers and implemented to support the gun. There is no rationale for changing that to a weaker support system or a completely different support system.
This is the base of the gun and I show the line of the two girders. Looks like your gun is supported on the fore part only. Another girder does not have any effect on the gun elevation.
And in terms of the support system you use, because of the girders having to avoid the levers coming from the gearbox, they impinged into the original driver’s position. Thus, the seat and the vision port were shifted to the left. The rear lever I show below is the problematic one in your system.
c. The driver’s seat position is too far back. The gearstick location is shown below and, also, the two extension parts at the base on the left side mentioned above would have made it impossible for the driver’s seat to be in line with the girder.
The extension points would have been behind the driver and this fits with the internal location of the girders according to the model’s torsion bars.
d. A couple of other comments…
Looks like a Luftwaffe radio inside? The Dragon Bison Pz II kit includes an FU5 system. Possibly correct.
Seats. It is unlikely that the crew stood while moving. So there would likely have been foldable seats attached to the walls like in a Stug or in this Bison version…
As you say, we can’t know for sure, but I think we can certainly make reasonable deductions from the photos and the Panzer II version. It doesn’t really matter as a modeller, anyway, if we just want to experiment with ideas in enjoying building our models. I merely raise the points I do in our discussion, NOT as criticism of your work, but for any others who are interested in exploring this field conversion.
I look forward to seeing your final diorama, too! 
) I will post also here, as well as my italian forum, this topic concerned this probably unique field realization of a s.IG.33 mounted on a Pz III, just like Nimrod are doing (so it,s unique or double ? 
) .
luckily 
