I have a new saddle ready to go on the road bike this evening, but its significantly longer than the old one it replaces.

So how to position it fore/aft relative to the old one?
1) Keep it centred over the old one, so small ‘growth’ nose and tail?
2) Align tails
3) Align noses
4) None of the above (please explain!)

4) Align the bit where your sit bones usually go. Not very precise, but keeps me in the same position, so it’ll do.

Personally, I would fit the new saddle somewhere near where the old one was (roughly option 1) then go for big ride with multi-tool to fettle en route.

Both the above. Place it based on sit bones to begin with, then go for a ride you don’t mind stopping to adjust the saddle on (i.e., not a club ride or one where people will get grumpy with you for faffing…)

1) if the roundness of the saddle is similar, then align the saddles where they flare from narrow to wide.

2) If the profile is very different, then you may end up sitting on it somewhere very different. In that case, ride on the turbo for a few minutes and then use a plumb-bob to see where you are in relation to KOPS. Then do the same with the new saddle and adjust accordingly.