I don't think I can move the tank much. I can probably get it .25" higher if I started cutting and filing. The only other option would be to move the whole tank backwards where there's more room and extend the tubing.
BTW, it looks like the needle valve on the OS 40 is at about the same height as the Super Tigre; the OS 40 is the "recommended" engine for the Avistar.

Has anyone used a Super Tigre with an Avistar?

Thanks,
Steve