tub spout - stange adapter

I'm trying to replace the tub spout, but the "universal" spout I bought at Home Depot does not seem like it will work. The pipe coming out of the wall is copper. Behind the wall I can see and feel that it bends and goes up to the mainfold. Where the pipe comes out of the wall there is about 3/4" of straight pipe, then about 3/4" of threads (3/4" diameter), then 1.5" of 1/2" diameter and finally a 1/2" non-threaded "nipple" with an o-ring. So there is about 3.5" of copper coming from the wall. If I cut off the threaded part of the pipe there will only be about 3/4" to use for a slide connection, which does not seem like enough. If I leave the threaded portion of the pipe the spout will not be flush to the wall. Any suggestions? Thanks.

There is no way a piece of "copper" would be threaded, and how does the reduction from the 3/4" thread to the 1/2" one take place. A picture would help considerably because you may be looking at part of the old spout, or the adapter for a Delta "pull down diverter" although that is usually larger than the sizes you are specifying. DO NOT cut anything off until you know exactly what you have and how you are going to attach the new spout.

Photo attached

I have attached a photo.

How much does the tub spout stand away from the wall? I would think you would cut from the pipe about a 1/16th more than it stands proud. If there are some threads remaining, you probably want to soften their edges so it doesn't damage the O-ring. If the O-ring doesn't sit back of the threads, I'd say the pipe is too short.

Edit:
From the pic, I'd say you probably need to use an escutcheon to build out the wall a bit unless that stub with the O-ring is supposed to be sweated into the threaded part.

The threads and beyond is part of a former tub spout. How to install a new one will depend on how is attaches and where it seals to the pipe.

Some plumbing fittings are designed so you can either screw them into a fitting, or, their internal size allows you to solder them onto a bare pipe. I think what you have there is soldered onto the pipe coming out of the wall and was part of the spout you removed. So, to install a new piece, IF the pipe sticking out past the threaded portion is nice and smooth and 5/8" diameter (the size of 1/2" copper pipe), and it is long enough, you might just cut off behind the section with an o-ring and use a push-on spout. But, if it is some other size, your best bet is to unsolder the threaded portion from the stubout and either solder on a threaded coupling to mate with a threaded spout, or a reducer and a stub to use a push-on spout.

you have to unsolder the "spout adapter" from the copper tubing where the thread is, or cut the tubing ahead of the thread and install a new 1/2" adapter for the new spout. This assumes a conventional spout. If your spout has a unique requirement, (i.e., longer or shorter piece of copper, slip on spout, etc.), then you have to make that adjustment after removing the existing spout adapter.