Forget the gray card and just use your OM's meter (which is really good). The only thing you have to watch out for would be very contrasty scenes, which will normally have underexposed shadows if you just use the meter reading (the gray card method is worse in this case).
For contrasty scenes, open up a stop from the meter reading (or two if the scene is really contrasty). I know this is counter intuitive, but with roll film that is developed one way only, it is the best way to hold shadow detail. Yes, the high values will be "overexposed," but B+W film has sufficient latitude for overexposure that you should be able to deal with the high values with paper grade changes.
Just so you know, I shoot LF for 99% of my work and use a hand-held spot meter all the time plus the Zone System. However, the advantage of 35mm cameras is their speed and flexibility. Taking time to read the gray card, etc., etc. effectively negates this advantage. Since the object is to get a printable negative, fast, and you have good metering technology built into your camera that is fast and reliable, why bother with slower metering methods (unless, of course, for educational or testing purposes)?
The disadvantage of roll films is that you cannot develop each shot individually; a "standard" developing time for the entire roll must be used. Therefore, using your in-camera meter, simply find an E.I. for your film that gives you adequate shadow detail for "average" (i.e., not too contrasty) scenes, determine the development time that allows easy printing from these negs, and go shoot. Less contrasty scenes will need a higher paper grade. Higher contrast scenes should be overexposed (as mentioned above) to make sure shadow detail is registered on the negative (because you meter reading will underexpose them in this situation), The resulting contrastier negative will need a lower paper grade to print well, but you will have adequate detail in the shadows. Practically speaking, this is as good as it gets with roll film. When in doubt, bracket. Small film is cheap and bracketing is fast and easy with your rig.
Hope this helps,
Doremus Scudder