Since helping my roommate put a jack plate on his new Z522, I've been thinking about my old man's 1997 391 XT. It needs a jack plate, just don't know how big of one. Any ideas?

A 5.5" setback would probably be just fine. If it's not you can always buy the spacers that bolt in between the plate and the motor. 

Another question. We recently had a battery drain on the crank battery. It runs the 200 Yamaha and the two lowrance HDS12 /HDS9. We're replacing the battery, but would it be worth running a fourth battery just for electronics? We don't want to rub them on the tm set mainly for potential interference. We do run both, but the 12 is only running when we're using the 200. In between, it's in standby.

Get the Sear's PM 1 AGM with ll50 CCA.  I run a 225 Opti with livewell, and 2 hds units.  No other battery has held up for me.

If the extra weight does not impede your speed or performance than go for it.

 

Personal preference.

I'd be sure to run at least a 29 series dual purpose battery with a 31 being a better option if you can get it to fit.