1 sourdough: you may be able to reduce the da pull on that 638 without clipping coil springs. I would first disassemble and check the parts for burrs or sharp edges. You can reduce the drag of the trigger trigger bar (or whatever they call the pat that houses the trigger recoil spring) by stoning off the sharp edges and assuring there aren't any burrs or sharp edges inside that bar as well.
If that doesn't satisfy you then I would get a new spring kit from Wolff Springs - they have 3 for the j-frames: Bullseye, factory replacement, and combat (I believe). The Combat is the heaviest, Bullseye the lightest. You can probably mix and match the recoil spring weight of your preference to the desired hammer spring weight to obtain a lighter trigger pull and positive, continually reliable detonation.
You will also need to practice with your double action pull as you will most likely not get the chance to cock the hammer on that 638. If your da pull is too heavy you may well pull your shots from one side to the other and miss your target even at close range. A smooth trigger pull will help keep you on target........
The same applies to the grips. The 638/38 come with horribly small grips. My 38 would 'roll' in my hand when it was fired because I couldn't tighten my hold sufficiently with the small grips - a set a Pachmyer grips fixed that problem. JMTCW and HTH. Mikey.