Easy (?) OD Auto Install

I’ve got a 904/8.75 with 3.55 rear on my 69 Barracuda.
Other than going with a Gear Vendors O/D, what’s the easiest overdrive transmission to install that involves the least cutting/grinding/fabbing/welding?
The Cuda I’m working on is too nice and original to take a sawzall and grinder to. I’m open to any ideas, even non-Mopar suggestions (I know, Blasphemy!)
Just trying to come up with a solution to drop highway RPMs on my driver.

Thanks in advance for any and all ideas.