I'm driving a '79 Chevy pickup with a 350 engine and an automatic transmission. It has about 110,000 miles on it and has a definite loss of power. Every weekend I pull my boat to the lake. Recently I've been having trouble making it up the hills that I never had a problem with before. I brought it to several mechanics and no one can figure it out. The tune-up didn't help. I hope you can help figure this one out.
The tune up didn't do it, so let's take it from there. First thing I would do is double check the ignition timing. Not only to verify the base timing being correct, but to make sure it's advancing properly, and the timing mark is not jumping around excessively. This may indicate a worn out, loose timing chain that can significantly drain horsepower.
My next check for a power loss would be a restricted exhaust system, usually the catalytic converter. The best way to check is by dropping the exhaust to get a good look inside of the converter. Sometimes you can bang on it and hear the broken up pieces rattling around inside. Another method is the use of a vacuum gauge hooked up to the exhaust to measure back pressure.
Another possibility would be the carburetor. It's hard to believe but today's complex electronic fuel injection systems are simpler to diagnose than many old fashioned carburetor setups.
I'm sure you would have noticed the high RPM and low MPH if the transmission was slipping. However, a bad torque converter, which links the engine to the transmission hydraulically, could cause a loss of power with symptoms similar to a restricted exhaust system. Not as common, but it does happen.
I was saving the worst for last. It may be that your high mileage 350 Chevy is just plain tired due to a loss of compression from worn piston rings along with a sloppy timing chain. An accurate compression test will help confirm or exclude.