Electro Coupler Question

Installing a LCRU2 on a 1990s F3 chassis. Converting magnetic to electro couplers.  Instructions show wiring positive to a single post and negative to ground such as postwar.  How do I tell which wire is positive? (see pic). Second question:  I initially bought 13/4"  couplers because that is all the supplier offered.  Later figured out that original was 2".  Will 13/4" work on 045 min work (they look better) or do I stick with 2" which I found in Lionel parts?  Thanks Art