RE: Can I temporarily use a 11 x 7 prom on my Magnum GP 40?
An 11x7 is also too much for either an Irvine 40 or 42 as well as the Magnum GP 40 and yes was an inappropriate prop to start with..
Agine if cutting down 9.5 x 7 would be appropriate for the GP 40 and only reducing diameter with no attempt to change blade planform or pitch. These involve additional skills and techniques and beyond the scope of what you want to do.
John