It actually sounds like your going the long way around to wind up with a .44 Special. Unless ........ what you really want is a dual cartridge handgun. Modern .44 Special and 44-40 usually use the same standardized .429/430 bore diameter. Having a 44-40 cylinder fitted to a 44 Special can be fun and interesting.
There have been several manufacturers who have made SAA look-a-likes in .44 Magnum. Those copies of the SAA basic "look" have all been of about Ruger Vaquero (original) size. Or about 15% larger than the SAA. They do look "Fine from the freeway" however and handle both 44 Magnum and 44 Special interchangeably without changing cylinders.
About that hammer question. I don't understand why it is even a question. Authentic, as applied to an SAA (Colt) means original 1st Generation. I'll risk life and limb here but, Anything else, including 2d & 3rd Generation Colts are just copies. Authentic need not apply.
The old sliding block, safety notch activated hammer safety looks a little out of place boy is mechanically a non-issue. Why spend money on a solution where there is no real problem. If you ant something to look "exactly" like a Colt, go buy a Colt in the first place.
Now, the two position Base Pin Safety for passing the BATF drop test is truly a nuisance and needs to be resolved. You forget, when you service the little ladies revolver and push that sucker in all the way, after the buzzer goes off, you WILL hear about it. For weeks. Don't ask how I know this thing. Hacksaw. Remove the last 3/16 at the recoil shield and re-polish.
If on the other hand, you really didn't want a dual cartridge handgun, ignore all of the foregoing except the the two position base pin.
Coffinmaker