Hi Josh,
The answer is yes, a Bisley grip can be, as you said, "fit" to it! It's not a drop in, but almost. I suspect you'll also want the Bisley hammer and trigger but it's not necessary.
Here's the process that I use:
ADAPTING ANY NM RUGER GF TO AN OM THREE SCREW.
THINGS TO KNOW:
In general: All Ruger SA grip frames including SBH Dragoon, Old Army, Bisley and Birdshead (excluding Bearcats of course) will interchange from the standpoint of the screw pattern from Single Six to Super BH, Old Model to New Model, and even NM to OM.
The hammer and trigger, all the screws, mainspring/spring seat/strut, and pins can be transferred over from your OM Grip Frame.
But if you want a Bisley hammer, you'll need a Clements Bisley OM hammer.
THE ONLY ADAPTATION NEEDED IS THE TRIGGER AND RETURN SPRING, AND THERE 2 OPTIONS:
1. You can use the NM trigger return spring on old model guns with a Clements adapter trigger. This is the easiest way and the trigger is wide to fit the wide trigger slot in all NM grip frames. This is required if you want a Bisley trigger. You'll also need the NM trigger return spring and two pins to use a Clements trigger.
___________________________Clements adapter triggers are the center two_________________________
OM trigger__________________standard____&____Bisley w/over travel stop__________________NM Bisley
They are $45 and are a 'drop in'. http://www.clementscustomguns.com/
2. Another option is to use your OM trigger and return spring/plunger. The OM Super BH triggers are wide to fill the wide slot in the NM GF, if you choose to use it:
You'll need a .22 LR case or make this simple adapter to fit in the NM GF behind the old model trigger. Both will need one NM trigger return spring retaining pin to hold your OM trigger and return spring/plunger in place.
THE GRIP FRAME EARS:
NM & Bisley grip frames will need fitting to match the grip frame 'ears' to the OM main frame.
Below are photos of the ears around the base of the hammer To simplify production in about 1997, Ruger increased the height of the standard cylinder frame mating surfaces 1/16" on both sides of the hammer to match the Bisley model frame ears so any style grip frame could be assembled to any main frame. All the Bisley GFs have tall ears. When you match the GF ears to your OM frame you can match them to the base of the hammer at the same and get a nice flush fit.
Bisley grip frames also need two special hex drive screws at the top of the grip frame:
http://www.brownells.com/handgun-parts/grip-parts/grip-screws/grip-frame-screw-b-back-ss-sku780001221-12078-28782.aspx?sku=780001221
Special fitting for Bisley grip frames: Because of the standard plow handle main frame angle on old models and pre 1997 new models shown below, that were made for low eared grip frames, fitting the top of Bisley grip frames to Old model guns and pre 1997 NMs can be a little more challenging as you can see. If you try to maintain the angle of the main frame, you run out of metal on the Bisley grip before you would ever mate the two together. The solution is to carefully decrease the angle of the main frame at the recoil shield. Of course both spots will require a little touch up with cold blue on blue guns, but is in such a protected area, there's little chance of wearing it off. Plus the original grip frame will still mate up properly.
Any questions, just ask. Hope this is helpful,
