I recently replaced VSS, TB, WP, and a few other items on the '99 SOHC 3.5l.

After a week or so of driving it I smelled and saw a water leak. Underneath there are green tracks coming down the oil filter. I haven't been able to trace it to a specific location. I'm HOPING it's something besides the water pump/gasket/o-ring, but I don't see anything obvious.
I didn't replace the o-ring b/c the one supplied with the pump was not close to the right one. I did remove the old one. It was still pliable. I cleaned the groove, oiled it. Thought it would be ok.

It's a sizable leak. The reservoir will go from full to add in no more than a couple days - maybe an hour or two of drive time.

Anybody dealt with a similar leak, or have a method of determining the source w/o just tearing back into it?

Even if I tear back into it, if it's dry, will I be able to find the source?

I'm leery of the new wp as a whole. It's the GMB pump that req'd grinding on the surface for the TB to clear.

I hope you didn't literally use oil. If so, the O-ring probably swelled up and broke seal. I normally use waterpump RTV on the O-ring back there.

I always coated o-rings w/clean engine oil before installation.
New or reusing.

Oil will cause rubber to swell and break down. I would begin checking there.