They may have been cut from cheap Asian AL stock. You should be able to find an occasional GTB chime box with the chimes for sale on eBay. But try posting in the Parts Seeking Thread on pinside first.

Also, try an email to Jimmy at pinball resource pbresource.com. You might be surprised; they have hundreds of parts they don't list on their website.

Looks like the guy might have all 3 of your GTB chime bars for sale:

ebay.com link: Gottlieb 6 5 16 Middle Chime Bar with new grommets pinball parts

Unknown of your musical talent... you could grind a small bit off each end if the note is low (or flat). If your unsure take both sets and visit a music store or a friend with a tuner. If it note needs to be lower the aluminum is already too short and your euchred.

Not all chime boxes sound alike.
I have two '70s wedgeheads side by side, and they both sound different from each other.

I've made my own chime bars and they sound perfect. Aluminum bar stock from Homedepot or Lowes. I just cut to the standard length for what the bar originally was and the sound was correct. If you had to, as RustyRazor says, grind a little off the end (which will raise the pitch) until you have the pitch you want. Frankly, I never had to do that. All you need is a hacksaw, a drill, and a file.

For anyone unfamiliar with how chimes work or why grinding might change the tone there are some short videos here to explain:

https://www.funwithpinball.com/learn/chimes-vibrations-and-pitches

/Mark

Never heard a horror movie tone from a Gottlieb. You didn't state if you checked each bar by itself out of the chime box. If they ring true that way, it's a binding issue - maybe their hole spacing.

There are many, many small things that can make a difference in tone. It is better to have the edges of the bar sharp. Chamfering the edges will change the tone in a bad way.

Did you check the chime box for cracks/ split seams?

Difference in thickness will absolutely make a difference in pitch! It changes the resonating frequency or natural frequency of the mass. Don't forget to bring your micrometer or dial caliper with you to Lowes/Homedepot! Sometimes the bar stock is mismarked or placed in the wrong bins.

Pick up a cheap guitar tuner and tune them yourself. Just make sure it's a chromatic tuner (which means it covers the full range of tones with all sharps and flats and is not just for guitar notes: EADGBE). Also, ensure that you can adjust the calibration up and down from 440 beats per second (standard "A" tone). This will allow you to fine tune / adjust it for the slight tuning variances with the existing chime bars (they all might be equally slightly sharp or flat from "perfect" pitch) .

https://www.amazon.com/Korg-GA1-Guitar-Bass-Tuner/dp/B002HPMTZU/ref=pd_cp_267_4/140-4293668-4039455

I've never tried tuning a set, but it'd be an interesting evening at the bench with a grinder, file and tuner. Not sure what note intervals they should be tuned to, but you could check your Bucanneer for a reference.

Korg_tuner (resized).jpg