RE: os 40 on canard
It might flood with the tank was that high but a worth a shot.
If you were to mount it inverted your prop should be mounted at the same level as it is now only the head pointed down which would lower your carb about 1/2 an inch or so. Sorry if I am missing something but it looks like you engine is behind the fuse so that is why I mentioned changing you engine mounting to an inverted setup.
Either way I feel that you real problem is with the tank level relative to your engine, is there any way you could raise your smaller tank to the spray bar level? I do not think the distance is your problem but the tank height try it first with a regular prop (for cooling) while the plane is secured on the ground. If you can adjust it and it runs in all ranges from low to high with no problem try it in the air.
You will be surprised at how much the tank level can effect the performance of a models engine. I would go with a bit longer fuel ine if it would allow you the proper height for the tank level. The mid point of the tank should be at the spray bar in your carb.
Another option is to go with a side winder mount the head pointed to the side this will also lower your carb.
Also keep in mind going with a 40 size will add more weight and require more ballast up front which will add to your wing loading and reduce the performance of your plane.