How to install 

To re-phrase Mike's answer - 
 
If your champ has a two-prong power cord, convert it to a three-pronger FIRST.
 
Now, for the slave connection, you have a couple of options - buy a DI box, like an H&K redbox, designed to accept a speaker-level input, or add a DIY line out.
 
The DIY line out needs a 22k 1W metal film resistor, a 270-ohm 1W MF, and a jack. A box, a roll-off cap and a 500-ohm pot in place of the 270 are optional.
 
Follow the speaker wires up to the chassis or connector and note which wire is 'hot' and which connects to GND.
 
Connect the 22k resistor to the 'hot' speaker terminal, and the 270 to the gnd terminal.
 
Connect the free ends of the resistors together and wire that point to the 'tip' terminal of the jack.
 
Wire the 'sleeve' terminal of the jack to the speaker's gnd terminal.
 
You can verify the wiring by looking at the schematic for one of the 135W Ultralinear Fenders (Twin, Dual Showman, Super) at http://www.ampwares.com/ffg/index.html
 
HTH!