To my knowledge there is no difference in the floorboard arrangement in a 350 and 390. Also I have a small dip stick on the opposite side from the main dip stick on both STEs ...you should too ...check it kinda hard to see them.
Yes port water pump is a PITA to get too and I hire a mechanic to change it. Only way to change that impeller on that engine and maintain your sanity is pull the alternator and bracket then pull the entire pump. My mechanic can do the job in about 1.5 hrs. Oil filter on starboard engine never been a problem .I place a board over stainer in front of engine and lay across it reaching with left arm. I break it loose with a cheap band type filter wrench use my hand to spin it off and spin on another. Works for me because I am over 6 ft tall with long arms...may not work for you but worth a try.