My oil pan is really dented and I bought a new one to replace the old one. on my 350 CID V8.

Replacing bearings is not a big job.  I always use Clevite.  You don't know what size until you take them out and check.  Undersized bearings are always stamped on the back.  It is best if you can mike the crank, but that is almost impossible in the car.  To do the main bearings, loosen them all a couple of turns to let the crank down a bit.  Then just push out the old and push in the new.  Use assembly lube or just coat them with oil.  Retorque the mains, and turn the crank until one rod throw is low enought to change them.  Repeat for the other three throws.  Retorque everything just to be sure. 

The standard oil pump is perfectly adequate for a stock or slightly modified engine.  A high pressure or high volume pump just wastes horsepower. 

Put a little silicone sealer in the corners when you put the pan back on.

As an afterthought...  I can't imagine a way that worn bearing could cause a vibration.  If the problem is in the engine, the most likely culprit is a failing harmonic balancer.  But a vibration is usually in the drivetrain.

Larry

How do you find out if harmonic balancer is bad?

Driveline should be ok, new pilot bearing, clutch, rebuilt 4 speed manual trans and diff and new u joints

While your in there, you may as well replace the rear main seal.  I did mine, so it has to be easy.