The stock replacement UCA's will have stock style bushings as well, which isn't want you want. I'd replace those brand new UCA bushings with a set of the Moog K7103 offset bushings. Otherwise you're not going to get much of anything for positive caster, which will make your car feel like you're driving a shopping cart. With factory UCA bushings you're unlikely to make it much past the "granny" setting on the skosh chart...
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And honestly the skosh chart was set up around using only offset bushings so even its caster recommendations are a bit on the conservative side IMO. I'd never run anything less than about +3.5°, which you should be able to get with offset UCA bushings, but not factory UCA bushings.
The Mancini springs I see on their website are listed as XHD's, which means they shouldn't be different rates and they should sit flat. The SuperStock springs are the ones that have different arches. Unless Mancini is doing something different, the Mopar made XHD's I have on my Challenger sit flat side to side.