Cable Install, Speedy or Difficult?

1977 CJ5, 304 w/T-18 4-Speed. I need to replace my speedometer cable, for a couple of reasons, 1st, over the years it's pretty much melted away where it's near the header pipe. 2nd, I think it's where I'm leaking most of my oil. It works just fine but it's just a matter of time before it goes, plus I'm losing to much oil.

I've not done this job before, can anyone tell me if this is this a simple or difficult install? Do I need to remove the entire gauge set-up from the dash, if so how? Or, can I just reach up inside and screw a new one on? Any tips would be greatly appreciated.

Thanks  
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Dirty Deeds DunderJeep 77 CJ5 304

Super easy job. You should be able to reach up from the back of the gauge. But why is oil leaking out of it? That sounds odd to me. Are you sure it's actually coming from the cable?  

Not sure at all. I'm thinking the speedometer oil seal may be gone. I know the rear main seal is always good for a few drops after running. I am trying to fix the easy ones first.  
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Dirty Deeds DunderJeep 77 CJ5 304

Does the speedo oil seal make any sense?  
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Dirty Deeds DunderJeep 77 CJ5 304

Nope. No way for it to pump oil into the cable.  

In the troubleshooting section of the Haynes Repair Manual it mentions the speedometer cable seal may be a source for an oil leak. I assume it means where the cable screws into the drive train, not literally out of the cable itself. Does this now make any sense? Sorry for the mix-up.  
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Dirty Deeds DunderJeep 77 CJ5 304

Yeah, but that would be right there at the connection on the T-case. Is it possible you have a clogged T-case breather and the pressure is pushing the oil out holes burnt in the cable?  

This is an area of the Jeep that I know how to use, general area to look for, so I guess anything is possible, I'll be checking the breather, thx again  
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Dirty Deeds DunderJeep 77 CJ5 304