When I bought my 66 Barracuda, the engine was damn near identical to yours, but set at 10:1.
It even had the same rearend/gear and transmission.
It was lazy out of the gate, but liked rpms, which is what I have been told about 340’s.
I wish I could tell you that I just installed a different rearend gear and everything got better. It probably would have…
I pulled the engine and built a 416…oops
I’d definitely install the 3.91 gears and see what happens. It would be good to know what converter you have.