Rear Main Seal Install

Guys,

I am struggling with rear main seal replacement on my 74 360 engine in my 69 cuda. I am doing it with engine in the car. I have the red viton one. The seal is about 0.050 inches proud or so when installed out of the box. It is way to big. My 68 FSM says to cut it off flush. I have read a lot of posts on this that say 0.010 in is max. It is very hard to trim that much off with the big steel retainer in the seal. I butchered the first one with a dremel.

How do I trim the seal?