The old B&M Hydro-stick was a modified dual range Hydramatic with a full manual shift valve body and a modified fluid coupling to let the engine rev up tighter without "stalling" where hydraulic pressure inside the coupling prevents any higher RPM. That transmission does not have a torque converter, which essentially doubles engine torque just before the input shaft of the transmission starts to turn, but the fluid coupling achieves an almost 100% RPM lockup, which a torque converter cannot do. 90% RPM efficiency is the absolute max a TC can achieve.
Rather than go to the effort of retrofitting a Hydramatic, why not look into installing a 700R4/4L60 4 speed overdrive transmission? They're much newer, easier and less expensive to rebuild, and adapters are available to bolt up that trans or a Turbo 350 (3 speed non-overdrive) to the 235 engine.
Jerry
