Having a great deal of experience with PTO's, drivelines, & mobile hydraulics please allow me to offer some suggestions. First of all you'll need complete information on the Garwood winch you intend to use, not just your understanding of it. Different brands and models all have their individual characteristics so this is primary. Armed with that information you'll know what the HP requirements of it are and if it requires a forward/reverse PTO or does it have an internal transmission to allow for the direction. If you need a heavy duty PTO (and you will) it may be rated higher than what the transmission can deliver. You need the driveline to fail before the PTO or truck tranny does because it is less expensive to repair if the power demands are substantial. I've actually seen the entire side of a transmission break off with the PTO attached because the tranny was the "weak link" in the setup. So obviously there is a great deal to "understand" about where your headed.
If you will allow me to comment on your statement of a "12K or 20K Garwood winch" this tells me you don't know what you truly need. So exactly what do you wish to do? There is a big difference in the two capacities. Is it because they are available surplus or do you have both of them? Garwood was a prime provider of winches during WWII, Korea and beyond. They are long gone so parts are an issue with them. Built under military contract often means they are different than commercial models and have excessive specifications as compared to the rest of the world, i.e. commercial application.
There is a possibility that an old mechanical winch could be run by adding a hydraulic motor but your pump may not develop enough pressure or flow to power the motor in that application. Your old pump will not have much pressure, maybe a 1000 psi if in good condition, as that is what the old dump trucks used.
Also you'd need a large enough reservoir and your hoist may not even have a reservoir. Also add a 4 way valve of adequate size, hoses, fittings, controls, etc. and now your spending a lot of cash. Here is another possible solution; if the winch does have a direction feature then perhaps you can run it from the current pump driveshaft by relocating the pump and run the sprocket/chain drive up to the winch. Just a thought but it would save a ton of work and lots of bucks.
I hope I haven't thrown cold water on your intentions but this is a hell of a lot more involved than most people ever consider. If you find my advice valuable and want to discuss further then you can always e- mail be privately.
