So I just spoke to a guy at Energysuspension. Obviously he's no expert in 1968 Chevy leaf springs so he could only really tell me that I have the right part that is listed.
I think if I was going to do it again, I would just get some new springs made up at 2 1/2" width all the way along - that way I could use the bushes straight out of the box, it'd fit the u-bolt plate just fine, and it'd probably be a whole load less hassle at the spring shop!
As it is, I've got some springs that more closely match the original 68's - 3" at each end and 2 1/2" in the middle, although the sexy taper of the originals couldn't be matched by the spring shops that I tried - mine have a step in instead.
I'm going to take the shoulder off the poly bushes so that they fit entirely within the spring eye, which is how I think the originals were - I've taken a couple more pictures for anyone who comes across this thread in years to come
Thanks again for all your help and advice