In the mean time, some questions and ideas to think about.
1. Do you have a long hose, and how do you stow it?
2. If the answer to (1) is NO, then you have NO RIGHT being in "pretty tight places" in a wreck. There is no way to effectively share gas with your team mates when something has gone wrong.
3. How big a sling tank? And is it the same mix as your backgas, or is it a deco mix? Are you diving a single tank for backgas?
4. If you are diving a single tank with a sling tank for redundancy, please realise that this is definitely suboptimal. You must limit your penetration distance and total distance to surface (measured in time) to the size of your redundant gas source. Most of the main NZ wrecks (Canterbury, Waikato, Lermontov) are too deep for a sling tank unless it is greater than 40 cu ft in volume. By the time you get up to the 63 cu ft and 80 cu ft tanks, the size of them really limits the effectiveness of getting into tight squeezes - you are way better off with a set of manifolded twins.
5. Can you deploy your long hose if you are carrying a sling tank on the right hand side? More importantly, can you do it in a confined space and in the dark? If you can't, consider switching your sling tank to the left hand side to free up the deployment of the hose - or better, moving to a set of twin tanks. Yes, they are expensive to set up - but how much do you value your life and the life of your buddy?