You won't find many willing to advise in much detail but this is what I would do, bear in mind however I have wreaked multiple barrels though jetting too lean.

Presuming you're happy it's not lean at the top I'd try the 40 slows with the needle and MJ as it is to see if that cleaned it up a bit, if not bring the main down a size or 3 several people have run '21s on o.e. pipes with the splices done and the airbox opened up on standard jetting I wouldn't be surprised to find that 142's are a bit rich.
Here's where having the jet kit really pays off.

That said the only way to find out what's going on in there under real load is to plug chop it.
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