I’m thinking about picking up a tracker 1542 Jon boat and building a casting deck on the front. My one concern is stability. I know wider is better for stability, will 42in boat be stable enough for me? I’m a smaller guy, I’ll be fishing by myself most the time. I plan on going on smaller lakes and trying to stay out of the wind if it’s bad. 

42" beam should be plenty stable for a deck on it...I'd worry if it was a 36".

It'll be fine with a deck. I built my deck literally on the top of the rails on my 1448. It's going on 20 years old and for years carried 2 or 3 folks with bows a gazillion miles of shoreline. Nobody ever accidentally got wet. Here's some pics.

I only have a 1040 jon boat (10' by 40") to use when im not on the kayaks and i can stand on the seat and pretend its a casting deck. So far no issues, but its a bit sketchy.

Ill update this if i end up getting soaked.

 

A 15'er with a casting deck should be plenty stable, just do some balance exercises if it feels odd at first. That should help alot. Maybe put something around the edges to prevent yourself from accidentally walking/falling off the flat sides. Its very easy to happen if a fish runs a certain way so you step to the side and you think you are in the middle.

If water is not choppy and your sense of balance is good, you will be fine.  I have a friend whose balance is awefull, he trips and falls on everything. Only you know how good your balance is.  30 years ago I would have been fine but at 75 yrs old, not so sure my balance would be that good.  On my 16' side console the deck is fine, but on a small Jon boat, I would not be comfortable but I am probably much older !!

You will be fine 🙂