C15 oil leak and transmission swap

95 FLD 120 with a 550 6NZ. Have had an oil leak that I can't find. My mechanic said it may be the front cover. Did a lot of cleaning to try to find where it is coming from. Cover looks pretty dry. The air compressor has its own filter for the intake. It was pretty clogged and oil soaked. Could this be where the oil is coming from?

This truck currently has an Eaton RTX15710C 10 speed with 4.10 rears and 295/75R22.5's. When this trans goes out or I need a clutch, I'm thinking of going to a 13 speed, possibly a double over. Would it be worth it? I usually don't run fast, around 65. I'm thinking this would put me around 1400 rpm at 65, especially empty. We have lots of hills here, so the 13 may be an advantage. My only concern was that I have only seen the RTOO in a 1450 torque spec. Not sure if this should be a concern.

 

That was my original thought. Just doing some research, and looking at some ratios. Didn't know if the .62 high gear would be any benefit over the .73 I have now.