I have the same dilemma. Got a new Shadrach this year and have been stretching it forever over a 21" diam. football, for my 16.5" neck, to no avail.
Sweet won't tell me anything other than to stretch it and not cut it. If you want to try stretching it first, they recommended using something inflatable-specifically a stern float bag- so as not the be pulling the gasket down over something large and risk tearing it.
So I asked my cousin who's a USCG rescue swimmer if he encounters this problem. He said he and his buddies go through this every time with new dry suits. He said they always stretch the gasket a little over a metal bottle (air tank) that's wrapped in tape then cut it as straight as possible with a very sharp x-acto or similar blade. Though, any round object will do. Try to cut it as straight and cleanly as possible between the ridges (any nicks can start a tear in the gasket).
Here's a pic he sent me to show what they use (smaller bottle is for wrist gaskets):