Engine deceleration issues

[69gmcc10]

I am currently working on a stock (to the best of my knowledge) 00 5.3l 2wd Silverado that I purchased to eventually transplant the drive-train of into my 69 GMC. Until then, I am trying to work out what little quirks the drive-train has. I have resolved most of the little issues with a through cycle of sea-foam and new MAF, but now I am working on the idle and don't know where to go from here.

The issue I am trying to correct now is that the engine does not decelerate very fast and when the trans is in park the rpms drop to 1000 mark and then they hesitates and stays there for a second or three before settling at 500. does anyone have any suggestions on what to look at?

I thought it was a leak in the intake gasket, but after allot of WD40 sprayed on the gaskets I came up with nothing. Also, the only codes the ecm has are intermittent rear O2 sensor failures on each bank. where do I go from here?