I was running my 70 455 442 down the highway at about 3300 RPM when the oil pressure dropped to about 0 and rapping started. No overheating occurred. I had it towed home and started it. It rapped at start but stopped as pressure was back at 50 psi. After a bit I could hear it starting slightly again getting worse as time went on. Is a bearing spun? Can I replace them from under the car by removing the oil pan?

 

Do you have an oil pressure gauge? This would help you determine if a bearing has been damaged but it certainly does seem like it from your description.

 

That or a collapsed lifter?
Nah, it sounds like a bearing.

Get used to the idea of pulling your engine. You're only kidding yourself if you think you can get it done from underneath.
You might have to cut the crank a bit.
How many miles are on it?

 

Pressure was running 3K psi and then dropped to 0 when rod rapped.

After total cooling and re-start, pressure was up to 50 psi and as car warmed up, I could here it starting again when revving the engine.

At idle, oil pressure is still at 30 - 50 psi

 

Does it sound like this?

When the 307 spun it's bearing

It's cure, a new engine, but it was a 307, not worth rebuliding

 

Nowhere near as bad as that.......

 

2 things you can do to get some idea. first, pull the oil filter cut appart and look for metal fragments ,pieces of bearing. 2 drain oil and look for metalic swirl when oil goes into drainbucket. Sounds like an engine pull is only answer.

 

Wow, now you just ruined my morning! After listening to that I am going to feel sort of sick in my gut all morning long........

 

Oh geez. This reminds me of when I let my oldest son drive my '70 SX while his car was in the shop being painted. It had only a few thousand miles on a new motor. He called me on his cell phone from his buddies house saying "Hey dad, your car is making this really weird noise. Listen!" I have no idea how long I futilely screamed "Shut it off!!" as he held his cell phone out towards the running, knocking car so I could hear it real good.