FWIW, I recently rebuilt the engine from my 71 Cutlass at 170,000 miles. Not because of wear, but there were a few issues that needed attention. The "straw that broke the camel's back" was a leaky oil pan gasket. I tried to fix just the pan gasket, but it was so much work I decided to pull the engine out. The job mushroomed into a complete overhaul of the engine and transmission. Good thing - because a lot of things got fixed that could have cropped up in the near future (brittle transmission seals, possible corrosion perforation of the timing cover and oil pan). The moving parts of the engine were in great shape, and could have gone another 100,000 miles.
If you have any other "issues" on your car, you may consider having everything done at once.
95 E320 Cabriolet, 131K