The ones with the pinned front sight all can have a big dot installed easily. Since you want to stick with the steel ones you'll need a newer 640, 60, or 649. To my knowledge, there were/are no steel Js with pinned front sights that are 38 Special only.
Also, the only one produced with the deeper, wider u-notch rear is the M&P340. I think the M&P342 did too. But, I have never seen one in person.
By the way, the M&P340 has a XS "standard dot" on it, not a big dot.