Oil pressure

A little background: why did you replace the oil pump? Did you replace it with a high volume one? What does the oil pressure currently read? At cold idle and cruise it should be around the 40's, or "in the lower middle" of the gauge. During warm cruise it should be a bit lower, and during warm idle it should be a lower end of the gauge, or in the high teens to mid 20's.

Did you take out the distributor at anytime?

What type of oil are you using? Sometimes using an oil of the wrong viscosity will cause abnormal oil pressures. Assuming the new oil pump and oil type are correct, my next guess would be severly worn internal engine components. Do the lifters still make noise?

The reason I ask if you removed the dist, is because it is what drives the oil pump. If the dist was somehow comprimised, then it could give the oil pump trouble.

Were you able to examine the bottom end of the engine when you had it apart to change the oil pump? Another thing that comes to mind is possibly a low quality oil filter.

I don't think I've heard of a timing chain jumping 180 deg. That's unique.

As far as the crank, you shouldn't need bearings of a different size than standard, unless you spun a bearing or otherwise had one fail on you in which case you would probably need to turn the crank to take out the scratches, and then get the proper size bearings for it. I'm not sure if a shorter stroke will give you more HP. The engine will rev quicker, but your piston displacement will be less.