It's no wonder there are so many issues with sealing rear main's

"It's too big"..... cut it!
"It's too small"..... silicone it!
"I don't want it to leak"..... silicone it!
"I'm not sure what to do"..... silicone it!

Dude, you have the wrong seal for the application. Don't care where you got it from or what brand it is it's either the wrong part number or mispackaged.

Do yourself a favor
http://www.manciniracing.com/su340remase.html
(description says silicone for whatever reason, it is in fact Viton)
Proper fit, no sillycone except where the main cap seals (if used) contact the rear seal.