When I replace trebles with singles on a spoon, is it necessary to use such long hooks, or can I just use a typical octopus hook?
I replace all my hooks on spoons. I like to fish heavier spoons in the 3/5 to 2/3 oz range for casting distance and to make sure I can touch bottom before the swing (mainly for Bulls, Coho and Steel). These sized spoons all come with monster 3/0 or 4/0 hooks that can do a huge amount of damage on smaller fish, so I replace them with barbless size 1 or 2's.
The type of hook is a personal preference, however, when replacing the hook with a smaller size and shorter shank, I was concerned that the spoon might occlude or blind the hook during the strike, because the hook is situated so much closer to the blade. Therefore, while I like to use a small hook I also like to make sure it is trailing the spoon at the same distance or preferable more than the original hook. As suggested above this can be done using Everyday's technique of using a length of Dacron fly line backing.
Details can be found here:
https://outdooraddictions.wordpress.com/2015/01/01/the-trailing-hook-method/So as long as the Dacron can feed through the eye of the hook you should be all good.
As an alternative technique, I replace the original split ring with 3 or even 5 split rings of ever decreasing size, with the largest ring at the spoon and the smallest at the hook. The use of an odd number of split rings ensures that the hook point will line up with the inside of the spoon (as per the original set up). As Clarki notes above, I am not sure if or why this should be important. With a small size 4 split ring at the hook end you will have no problem running it through the eye of most regular (non Siwash hooks), with no stiction or restriction in the free movement of the hook.
You can buy packets of split rings at West Coast Fishing Tackle on Hastings (and all the other components to make you own spoons) and I would recommend a set of split ring pliers from Michael's Craft store to save your nails.
HTH