Welcome to the forum! Grips that fit any model-numbered round butt J-frame (like your model 36) will fit a model 342. The only J-frames that used a different size grip (other than square-butt models, of course) were the very first ones built in the early 1950s, which used shorter I-frame size grips. Later in the '50s S&W lengthened the grip slightly, and that same grip size has been used on all round-butt J-frame models ever since.
If you're looking for a long, hand-filling grip for the 342, I'd suggest a set of Uncle Mike's combat grips. They're very comfortable, and should fit your hand almost exactly like the Bianchi Lightning grips. (I used to use those, too! They weren't very attractive, but they were extremely comfortable and made shooting the J-frame a lot more enjoyable.)
Here's a set of Uncle Mike's combat grips on a model 042:
Another option would be Crimson Trace LG-305 lasergrips. I just put a set on another J-frame this weekend, and found them to be nearly as comfortable as the combat grips pictured above. They're a bit smaller, but still allowed me to get all three fingers on the grip.