There were a couple of 3-speed ODs that will work. The older T85N is referred to as the Borg Warner, the later (65-72) is the Saginaw although both versions used a B/W overdrive unit, and even though some parts are shared they won't interchange. By interchange I mean you can't bolt a Saginaw OD tailshaft to a early 3-speed & vice versa. But a complete later OD trans will bolt up to your 235 and has the advantage of a synchro 1st gear. Car and truck versions of the Saginaw were the same (to my knowledge) so that shouldn't pose a problem but the shift rods on the tranny side were different from the earlier unit, so some fabrication may be needed.
The factory scenario on both these ODs is hard to duplicate these days. It was actually a fairly elegant design, with a governor to prevent lugging (certain death to overdive internals), a kickdown to give a "passing gear", free-wheeling, clutchless shifting, and a momentary ignition cutout for smooth engagement of the OD. Very slick and almost seamless. But the parts for the entire setup are hard to come by these days, thus the need for alternative solutions like that mentioned from the Burb club page. This article is well done and reflects a thorough understanding of the way the OD was designed. But, a simple toggle switch to control the solenoid is really all that's needed. My experience is that split-shifting through six speeds is fun for a while, but as the goal is top gear cruising, I just engaged the solenoid for the OD once up to speed in high gear.
True overdrives went out of fashion on American cars in the '70s, once Borg/Warner perfected the manual OD 5-speed in a lightweight, compact but efficient package. Much less costly to produce and the end result is the same, although features like free-wheeling and clutchless shifting are nice to have, once you get used to them.
