How much leaner with headers?

So long story short. I put a FAST AFR datalogger on my '64 dart.

(Mild 360 4spd. Stock 1.88/1.60 Jheads, LD340 Street Demon 750, Comp Cams 268AH10, 9.4:1 actual cr, 340HP exhaust manifolds, 2.25 head pipes, 2.5" after the muffler)

Anyways, at idle I'm 13.5-14.5:1, cruise 11.5:1 and at wot it's as rich as 9.5:1.

So my question is, without changing anything else, how much leaner would I get by switching to some TTi long tubes and a full 2.5" exhaust?