My four (approx 1970) Gibbs likewise - drums turning on bare shafts - two bottom-handled for sheets, two top-handled for halyards. The sheet winches get a smear of winch grease on shaft and pawls each winter. I doubt you'll ever need to shim them. On one of my sheet winches I've had to polish the shaft down with emery cloth to make it a little looser (I think the drum might have been replaced a while ago). My problem was broken springs, but there is a brilliant springmaker who made me spares for all three patterns I needed - Airedale Springs http://www.airedalesprings.co.uk. No connection, just a very happy customer.
Best to use 3 in 1 (toolbox in a can) on the pawls as grease can cause them to stick.
It doesn't. I've been using the winches for 14 years without any such problems. I use a light smear of winch grease on the pawls. It's not precision engineering, so there's a big gap between the pawl and the pin it turns on. The grease also reduces the chances of the pawl going over the side when reassembling or dismantling the winch.
I sent them intact springs, albeit somewhat tatty. I also sent photos so they could see how the springs fitted on the pawls. They would probably have managed with a broken spring, but better to work from an intact one. I needed to send three - pawl springs are the same on both types of winch, but the ratchet pawl springs are different on sheet and halyard winches. My three little plastic bags are probably about 50 years' supply of springs, but there's no point buying just one or two when the spring maker has set his jig for your springs.