I'm with you on this Yorky, what a pain this can be! It took me nearly a week to get mine back on as the rubber hardens & shrinks slightly with age, so here are my tips;
1. Obviously, it's a lot easier with two people
2. If you've got a heat gun, warm the stubs slightly (& carefully) to get them more malleable.
3. Remove the air filter from the housing and use a large allen key or any other tool that can reach through the tunnel to apply pressure to the stubs from inside the manifold.
4. As stated by Lynx, get the stubs onto the carbs first, rotating each of them to get the best angle
, then you can reach through the tunnel with your fingers to pull the rubber seal over the tunnel mouth of the air filter housing.
And the very best of luck to you!