Believe it or not, a lot of us bigger-hand guys (I'm also 6'4") prefer a "pinkie under" hold if your pinkie knuckles are flexible enough.
Basically, I shoot with my pinkie curled deliberately under the grip but NOT "free floating" - I'm holding it firm under there. As the recoil takes the barrel up, the bottom edge of the grip "extends" my pinkie. Said pinkie is used as part of the process of slowing that recoil - and then curling the pinkie helps bring the barrel back down from recoil.
This process also helps position the gun the same way in the hand for each shot - what's sometimes called "indexing".
The bottoms of each panel on my Ruger New Vaquero (same size as the Colt SAA) have been rounded rather than leaving the two flat beveled panels in place.
My hands are somewhat big, but not enough so that I have a comfortable thumb reach to the original hammer with this pinkie-under hold. So I run a lower-slung SuperBlackHawk hammer to reduce thumb reach.
I recommend trying out this "pinkie under" type of hold. If it doesn't work out, then the Bisley grip is probably your best bet.
In all cases, I recommend sticking with Rugers. You can swap grip frame parts around and tune any given gun to your needs.
Ruger grip frames in a nutshell:
XR3-RED: this is the standard grip size of the Blackhawk post-1966ish and SuperBlackhawks with barrels of 5.5" or less. It was also the normal grip on the "Old Vaquero". It's a slightly oversize variant of the Colt SAA "plowhandle" grip. NOTE: because this grip frame was so common, there are a hell of a lot of "oversize" grip panels that stretch or expand this grip frame as needed. Aluminum grip frames in this size are common and dirt cheap (less than $30) and can thus be used to lighten any Ruger.
Bisley: Ruger's "Bisley" is really descended from a custom gun of the 1930s known as the "#5", owned by Elmer Keith. Ruger's version is oversize from the #5 and is noted for being able to handle big recoil. Usually matched with a very low-slung hammer and trigger with more curvature than normal.
The Bisley hammer/trigger sets can be matched to other grip frames as desired.
SuperBlackHawk: SBH Rugers with a barrel more than 5.5" come with an oversize plowhandle grip frame bigger than the XR3-RED. Most have the "squareback" trigger guard often noted as being the stupidest idea Ruger ever had (the "knucklewhacker"). Ruger's "Hunter" series SuperBlackHawks have the big grip frame but with a rounded-back triggerguard that is highly prized for those with big hands. The "Hunters" also come in a Bisley variant. These are possibly the best Ruger SAs ever made - their heavy 7.5" barrels come with integrated scope ring mounts and with Ruger's excellent scope rings in the box. Damn fine guns.
XR3: Ruger's original 1955 - 1965ish grip frame was very similar to the Colt SAA grip frame, enough so that Colt grips could often be fitted to the Ruger. In 2005 Ruger brought this size back with the New Vaquero and both 50th Anniversary "Flattop" limited edition Blackhawks (357 and 44). This new variant always comes with a keylock hidden under the grip panels, and so far the only factory grip panels are black checkered plastic.
Bird's Head: currently not in production, but available aftermarket, this smallish round-back frame is actually pretty respectable at recoil handling, at least up through strong 357Mag power levels and for some people, up through at least moderate 44Mag horsepower.
For really big power, the Bisley is now considered king of the hill. John Linebaugh now refuses to ship his monster 475 and 500 customs with any other grip frame. To get an idea what level recoil those entail, see the video at:
http://www.customsixguns.com/
If your hands are truly gi-huge-ic, you may need to go to the SBH grip AND do pinkie-under! That would be what I'd try if my hands were too big for the Bisley.
