Your bushing engine is quite a bit narrower than the evolution .46 BB engine that is typical on that trainer. I don't really like how that looks. Here is my suggestion--go and buy an evolution .46.

Naa, just kidding.

Now, for real--use your great planes adjustable mount. Take the original mount off of the firewall. Take the blind nuts out of the back, and fill those holes with dowells--glued in nice and tight. You may have to drill out to the next size in order to get a dowell in--get the smallest dowell that you can get that will still be snug! Then, re-drill for your other mount. Make sure it's all centered up like it should be--I'm not sure where the thrust line is on that engine. You might want to consult with horizon on that issue.

Getting the blind nuts back in will seem an impossible task--but if you have a telescoping magnetic tool, that will do the trick of reaching up in there and putting those nuts where you want them!

Hope this helps..

Edit:
BTW, others might say that your setup is fine as is. It really all depends upon how comfortable you are with it. Can you grab the engine and pull as hard as possible? If so, does it budge? If not, then you are probably fine. Just check the tightness every so often. I'm just speaking for my preference--I wouldn't like it....