MIMF

In a previous post I mentioned that I was moving from finishing all my repair jobs to starting new build projects. What I'm going to be working on is a 8 string (as in 4 root & 4 octave strings) 28.5" short scale bass. The neck I'm planning on using was starting life as a 5 string but has had the tuner holes drilled, filled, & re-drilled in different positions. I also don't think the head is large enough to accommodate all the tuners. I'm thinking about cutting off the head the gluing another piece or two of maple on large enough for all the tuners once shaped. My concern with that would be the strength. If I were to cut & glue new pieces as illustrated in the picture below would that have enough strength to withstand the tension of the strings? If not what would be a better way to do it?

You could use splines or biscuits in the joints to strengthen them and put a one piece faceplate over the lot.

... and backplate too.

Yes, I thought that after I posted to add a faceplate and backplate to increase the strength. I'm thinking that as long as everything is glued up securely all should be fine. Would you all agree?

What style of head-stock is it? Flat like fender or angled back like Gibson?

I think it will be fine, as long as your joints are perfect and you use a quality glue. You should also come up with some type of jig or clamping arrangement to apply sufficient clamping pressure on these non-parallel surfaces.