i prefer the round butt K-frame due to the wider selection of grips available.
i once had a m-13 sq-butt that i wanted to convert and found that there is definately a "quick-n-dirty" way and a much "nicer" way.
q-n-d (actually the workmanship was very nice, just not very "factory" looking) consist of:
1. placing a set of rd-butt "magna" grips on the frame
2. scribing the outline
3. grinding the frame to match the grips
4. finishing off with files/stones
it gives you the right shape, fits all the correct grips and holds a tight budget.
the "nicer" way added:
1. completed/replaced serrations removed during grinding
2. re-blued frame
much nicer to looking, but not functionally different