Steve,
S&W did make the 940 which is very similar to the 640. Put the two side by side and it is difficult to tell them apart. The 940 chambers five rounds, so a seven round cylinder would be considerably larger and result in a larger frame.

S&W did make a lightweight 942 for Wiley Clapp and Mr. Clapp said the recoil was very snappy. Several members of this forum put 940 cylinders in 642 frames with good results. You can do a search and you will find some interesting reading.