There is a very marginal difference between the Bulldog frame, and the J frame. Not even enough to make a difference.
Quantify that diffence. Then square it.... that still doesn't equal the imortance of the critical dimension here: The 6 shot .38 clyinder is TOO FAT for the j-frame. Maybe not enough for you to eyeball and consider significant, but S&W's engineers tried and could not make it work reliably and safely .....
... and if it were possible, AND here was a demand for it such that further R&D would be rewarded with a nice profit, then the market would have made it happen, don't you suppose?