Hi Steve, I've never seen anyone convert a Toon into a Stug III so you might be the first, seen the long-barrelled 7.5 cm KwK 40 gun convertion done so good luck I'll be whaching this one with interest.
His may or may help.
Dry Fitting Test Tip.
The only downside is that dry fitting is a problem as once the two parts are together they are some times very hard to separate. I soon learnt that early on with the toon tanks, I should have mentioned it, far be it for me to tell any of you how to build but a quick sand around the connecting pins and you can get them apart again without damage, the other tip is to cut 3/5mm of the smaller connecting pin the male one, that is best one I have found.
I would also recommend that you thoroughly wash the tank tracks, on the last two toons thay were very oily and even the sprues had a slight oily feel.
I haven't come across a glue that won't glue these together yet and I glue everything on.
Pete.