My new (cheap) water pump appears to be leaking at the gasket. I am guessing this is partly due to vibration from a loose alternator. But also, that surface of the B20 block is pitted where the gasket goes. Like maybe this was a marine engine and salt water went through it. I tried filling the pits with JB Weld, but something has let go and I suspect the pits are back. The temperature at the time was very cold for JB Weld and it never seemed to cure right. My question is, can I just take a piece of thin sheet metal and carve the profile of the water pump body (outside only) out of it, and lay this against the block? then mount the water pump (with gasket) against the sheet metal? Seems like it might help me avoid the pits issue.

I would suggest that you re-apply JB Weld at the recommended temp. Sand cured Flat to the block surface. The stuff works.

In you don't want to do that. You may want to try this on the Block-side of the new WaterPump gasket. Apply a thin layer.
Unlike silicone gasket compounds that come in tubes it will not squeeze out an risk clogging the cooling system

https://www.permatex.com/products/gasketing/gasket-sealants/permatex-aviation-form-a-gasket-no-3-sealant-liquid-3/

Unless the corrosion problem is severe a good gasket with a reasonable coating of a good sealer should stop any leaking at that point. The more usual leak point is from the rubber ring seal that goes between the pump and the head. Correct thickness must be used and any corrosion in the head recess removed. This recess not so easy to clean when the head is on the engine. You can use a bit of sealant on the seal or silicone grease. When fitting the pump, leave the bolts a little bit slack and use a length of 4 x 2 underneath with a floor jack and put some upwards pressure on it before fully tightening the bolts. Make sure you don't squash and distort the seal when pushing upwards or you will be doing the job again after it starts to leak. The bolts need to be slack enough to allow the pump to slide upwards a fraction.

oops, sorry for the vacant post. Thanks Derek and CB. The problem went away after the alternator got calmed down. I will keep your suggestions for if the leaking comes back.