Tim brings up some very valid information we really need to know to properly advise you. However, just for basic information there is some things we can tell you. Where some of the confusion you might be getting is that there are several different transmissions that were used with the overdrive unit, as well as 2 different overdrive units themselves. The overdrive part of the transmission was made by Borg-Warner, regardless of what the main transmission was. The 3 common transmissions it was used with were the SM318 (SM319 when used with the overdrive), commonly referred to as the 4 bolt Muncie, the SM 326, which is the 7 bolt Saginaw, and the Borg-Warner T85 (T85N with the overdrive. The first 2 are the ones used with GM vehicles, the T85N was primarily a Ford/Studebaker/Jeep transmission (the T85 was used in GM trucks as the HD 3 speed option, but not with the overdrive), though I have heard of some late '60s Pontiacs using them, too. The SM319 is the one used in the '55-'66 GM cars and trucks, and is basically the same as the standard 3 speed other than the overdrive unit itself. The SM326 was used in the later vehicles, is fully synchronized, and is a bit longer than the SM318/319. It also has the stronger R12 overdrive unit.
I would think you would be OK towing a small boat with any of these transmission, but you will want to bear in mind that with the overdrive active, you will not have any compression braking below 28 mph due to the over running clutch in the overdrive unit. You should also be aware that most of these overdrive units have speedometer gears in them calibrated for 4.10 gears, so you will need to recalibrate the speedometer, or drive off a tachometer like I do.
