Re: 1972 18 horse eletctric start
punch these item numbers into ebay...
120531127884 This is the one that fits your motor. It has that flat angle.
130367994891 This doesn't. It has the solenoid mount. You can see where the solenoid is, pay attention to this area.
260555775522 Here's the bracket that doesn't fit. another difference is at the top. It has a tab. The one you want doesn't have a tab like this.
As for wiring, + battery cable goes to solenoid, - cable is grounded, usually with the screw on the starter bracket, a lead going from solenoid to bottom of starter, and two wires at the top of the solenoid going to a mommentary on/off switch