Apples,
Converting the NM Flat Top BH to just the old model action w/o transfer bar is pretty simple. If you haven't had a NM apart yet, start with these factory videos at the bottom of this post. You'll also become familiar with the part names and what they look like.
NM converted to a 4 click OM action w/o transfer bar or loading gate lockouts:
The Power Custom 'Registered' old model style hammer has a safety notch, a ½ cock notch, will give you 4 clicks, and would be the easiest approach. Use with their modified trigger or modify yours. The very thick Ruger trigger sear must be thinned down from the front side (like an old model Ruger trigger). The rear corner of the trigger, which is the actual sear is not touched. That's why PC sells the modified trigger, but it's so easy to do, it's not worth the cost IMO.
Just leave out the transfer bar. The loading gate will be freed up so you can open it with the hammer in any position.
Other options are an original old model hammer or the PC Bisley hammer made for old models.
Using the PC registered hammer, the PC Bisley hammer for old models, or an original OM hammer is almost the same. None can be used with a transfer bar because they have the flat face. All three need the trigger modification as described above to fit in the notches.
To use the PC Bisley hammer 'for old models' or an original OM hammer, there's two additional modificationsrequired:
1. Shim the screw hole, because the OM screw is slightly larger than the NM pin. And that's simple: take a 22 case with bullet and powder removed and primer fired which is just the right size, (a fired 22 shell won't work because it's expanded), press it into the OM hammer screw hole in a vise, and cut the case off flush on both sides. Debur the edges and clean out with a 22 bore brush. Now the NM hammer pin will fit perfectly.
2. Cut the transfer bar connecting boss off of the trigger to clear the hammer bases.
With any of these hammer options, you must cut off the shaft protruding from the cyl latch so the loading gate will be freed up to cock the hammer with gate open.
NOTE: On the New Vaqueros and Flat Tops you can have a free wheeling cyl as well, just remove the set screw, spring and indexing pawl in the hammer channel.
Factory Video - New Model Ruger Disassembly:
http://www.youtube.com/watch?v=UfOXBa8K8Ow
Factory Video - NM Reassembly (shows hammer plunger assembly too):
http://www.youtube.com/watch?v=3zEKDqwkgEs&feature=related