Since there are a lot of you choosing the 135gr Speer Short Barrel .38 Special +P round, why not the 135gr Speer Short Barrel .357 Magnum round. It is the best .38 Special +P load available. (I'll explain)
The .38 Special +P is rated @860 fps from a 2" barrel
The .357 Magnum is rated @990 fps from a 2" barrel
Since the SB .357 Magnum load is just slightly more than a good .38 Special +P why not carry them in your M640? Both use the same bullet which was designed to reliable expand at the lower velocities associated with a short barrel but the additional 100 fps from the magnum round is just a little insurance. IMO it's a win-win without using a full power magnum load that's too difficult to shoot well...
And since the case actually fits the gun as intended they keep the chambers cleaner, and since the OAL fits the gun as intended, there is less rifling gap so the round will be more accurate.