don't want to sound bossy, but you need to sell the entire 28 set up as a kit. keeping the lighter and shorter 28 type slides to use with later caps, i believe, if i recall correctly, won't interchange with replacement parts and frankly makes no sense. i know this sounds like i'm being a jerk, my apologies in advance, i don't have much time at the moment, so i'm just going straiqht to the point.
if you split the set up, you will devalue the set. you say you want to keep slides and springs and sell the caps and cable as a "complete set." how do you figure you are selling a "complete set" if you are keeping the slides and springs? and those old "lighter" 28 type springs are weak and probably worthless from the standpoint of using those springs with the light slides for a rider.... assuming that's your plan.
again, not trying to sound like a jerk, i just don't have much time to write this, pecking away on my smart phone.
if you sell everything related to 28's, it's probably worth $1k +/- on ebay. maybe more. split it up like you are planning and it's worth less. you can (or used to be able to buy on ebay) the kits or the pieces to make up the replacement kit. it's been a while, but i bought off ebay a couple of these complete kits for around $200.
in fact i have a full assembled complete very good used condition set up just as it was taken off the bike (caps, ring nuts, cable adjusters, slides, springs, needles, cable, etc.) i'll probably never use if you are interested in paying what i feel a reasonable price (after you sell your 28 kit of course. if you are interested in purchasing what i have, please feel free to pm or email me.)