You could try your 3.89s with a Gear Vendors overdrive. That has an 0.78:1 ratio, and makes a big difference in cruising rpms (a 22% drop.)
I have that setup in my 4-speed KR (3.50 rear) and love it on the highway. But you still keep the regular non-overdrive performance everywhere else.
Pretty straightforward to install. You will need a shorter driveshaft, though.
Did you shorten your original driveshaft or get a new one made? How do you control it with a stick?
You could do either, but I had a new one made.
I have a Hurst t-handle shifter, with the button on the t-handle. I put the control box in the glove compartment. It has red and green lights to indicate whether you are in O/D or not. When up to the mph that I want O/D, I push in the clutch, push the button on the t-handle, let out the clutch and you're in O/D. Simple. There's also ways to split gears, 1st low, 1st high, and so on, but I never do that. I just wanted reduced revs at highway speeds. Works great for me. Check out their website for all the info.
I've used a GearVendors in 2 other cars and they work great, but those were both automatics. Same purpose as you, not to split gears, but to reduce RPMs on the highway. I wasn't sure how you'd do a clean install with a stick shift though. With the automatic is goes on automatically, and I have a floor button (high beam style) to manually punch out if I want more speed.
I don't have a hurst handle, so I'd have to try and do something else creative... Could go with a floor button, but would be great to have it on/near the stick.