I have some knowledge of these units. There is only ONE basic unit, all are 30% overdrive. The only difference between R-10 units for each manufacturer is the 1-1.5" think adapter between the trans and OD and sometimes the output shaft splines, though all I know of are the same.
There is only one thing I see "wrong" with your application. The planetary gears are rather fragile until fully engaged. All factory systems momentarily ground out the coil (effectively turning off the ignition) when disengaging OD. This is CRITICAL! I had one mounted behind an EFI engine and decided the electronics might not like grounding the coil, so I wired a relay into the ignition wire. The kick down switch momentarily cut the ignition (faster than you can turn it off and on, but similar in concept -- maybe two cylinder firings at the most). That is just enough to take the load off the input and keep the unit from binding inside. If it binds to much the shaft one or more of the planetary gears ride on will break and the OD unit will jam. Luckily this doesn't leave you stranded, it just jams the unit in direct drive as if the lock-out handle is pulled.
One more thing, these units automatically go into OD once 25-30 mph is reached. The goevernor is nothing but a rotating weight that trips a set of contacts to complete the circuit and allow the solenoid to engage OD. I suspect you are using a simple switch to energize the solenoid when you want to. That's fine, there is nothing wrong with doing that and splitting gears in second and third, but 2 and 3 are so close together in a normal three speed trans that 2 + OD is about the same as third. It would be worse with a four speed -- 2 + OD would be a little between 3 and 4, which would make for some fancy shifting! The unit was intended to just be a high gear, though AMC did make a three speed trans with wide 2 to 3 spacing so it could be shifted as a five speed for a few years. Not a very fast shifter though (they called it "Twin Stick" because the lock-out was on a short shift lever beside the three speed floor shifter insted of a cable under the dash). But you HAVE to either clutch the trans coming out of OD or install some sort of ignition cut-out in the kick-down switch!! I wired one up sort of like I think ou did, just a switch for power to the solenoid. It worked fine for a while, then one day it bound up and popped a planetary shaft. This was in a little 61 Rambler Americna (2600-2700 pound car) with a 90 hp (gross hp!) flat head six!! The same unit is used behind 390 Ford big blocks.
You could have a problem with the auto shift with a big engine. A friend of my dad's used to race a 50 Ford with a 312 T-bird engine (built!) and three speed with OD. He told me about breaking the OD on numerous occasions, usually out drinking and street racing. When OD is engaged but you aren't in OD yet, there is a chance of applying to much power as the gears are engaging. Basically he was taking off just cruising, just at the OD engaging speed when someone pulled up and gunned it. If he forgot about OD not being locked out he'd let off the gas slightly then floor it. Well, when he let off the gas (either on purpose or by mistake) and suddnely floored it the car took off, usually with a big BOOM under the floor from the planetary shaft busting!! Yep, sounds like a shot run going off under the floor when that happens, scared the crap out of me the only time I had it happen! I was crusing in OD when the shaft in mine let loose, it had weakened from all the driving I'd been doing I guess.