Since I'm away with work just now, I'd gotten a friend to have a go at fitting that new front propshaft I got.
Turns out the new one looks like it's about 2" shorter than the original - I'm away so can't check myself - would any of you kind souls know the right length so I can chin our parts shop.
You are not alone:
KJ Font D/S Length 16 1/2" 0r 19" - JeepForum.com
Looks like a 30 inch collapsed length is needed.