Clutch adjustment is usually done on these cars via turning nuts and/or threaded rods to adjust the length of the clutch pushrod- found under the hood above/around the steering box. Have someone push the clutch pedal while you watch what moves under the hood and it will be very clear.
The most critical adjustment is for an adequate amount of "free play", the amount you can push the clutch pedal before you feel resistance, where you are noticeably engaging the clutch. If you have little or no free play the clutch can slip and burn up. I personally like a clutch that engages lower in the pedal travel so I adjust for more free play than the manuals specify. This is not a problem so long as I don't adjust it so low that it doesn't fully disengage when I push the pedal near the floor. You will know if you have it too low if the gears clash when shifting and/or it's hard to get into gear.
Your odometer likely has 123K, 223K, 323K or more on it. When they reach 99,999 they roll back over to 0. That's why on these old cars you have to take general condition into account. A museum piece with 20K on the odometer MAY actually have 20K but an old worn car usually has had 2 or 3 odometer rollovers over 50+ years. Years ago when these cars were not likely to have more than 200K, I used to help tell the difference between a 60K and 160K car by looking at the rubber pads on the brake and clutch pedals- they lasted about 100K miles or so before wearing through- so a 160K car would have worn through pads or brand new ones
As far as how long a clutch would last- I think 60K-100K miles was normal depending on gearing, the local terrain, and how the car was driven. Someone living in downtown San Francisco would likely get on the low end of that or less, someone who drove on mostly flat freeways could get much more.
Hope this helps a little.