Not many pontoon boats in my area.----But pontoons are generally slower through the water and may not develop as much water pressure.----Hence the different spring.-----Your local Mercury dealer will likely help you on this.
There must be a --Chriscraft ----Owners group.-----Or Antique boat club online.-----Many of those folks are very helpful.---Boat museum in Clayton , New York is also a source.----Was boat properly winterized ?
I believe that site has the exact wiring diagram for your motor.----But do a complete evaluation on this motor BEFORE spending beer tokens on it.-----And --"great compression " -- means nothing to me.----Post the actual / accurate values.