So, my truck already has a liquid cooler for the transmission in the radiator. I want to add an air cooler for that and add lines for the engine oil going into the old oil cooler in the radiator. Does this sound like a decent idea, or if not, how would you guys set it up?

 

I would not run engine oil through the radiator. Are you towing, and that is why you think you need to cool the engine oil?

Back in the early 70's, Ford had oil coolers on some of the big block hot rods of the day. You could get one of those adapters(screws on before the oil filter) and then plumb a small cooler to it.

If you have issues with transmission temps, I would just add a big cooler in front of the radiator.

 

On my 302 and 351W with E4OD this is what I did. Yes it is a good idea to add a better auxiliary cooler for your transmission.

At first I used the Haydens oil cooler kit. For both my auxiliary transmission oil cooler and my primary engine oil cooler, "that had the plastic ties that went through the radiator." They seemed to work okay for my application at that time. I still burned up my transmission a few times in the Texas Heat with the Haydens transmission cooler attached..

I now since fabricated a steel bracket to my radiator support using slotted square steel tubing, lock nuts, washers, bolts. I drilled the holes in the support and mounted the tubing... I cut 4 pieces of tubing. Two pieces that ran more then length of the radiator...I had to notch those tubes for the hood latch support bracket to run through. Then the other two pieces of tubing I had those mounted going up and down that I bolted to the radiator support.

I still use the cheap Haydens engine oil cooler with plastic ties because the engine oil temperature gauge barley runs 210 degrees on the hottest days of summer...That's after running 85mph down the highway for about 50 miles. Using a 190 degree Thermostatic oil sandwich cooler adapter made my Mishimoto. So a huge fancy cooler for the engine oil cooler is not needed in my opinion. The temperature gauge is in the Oil sandwich adapter along with another oil pressure gauge.

Although, for the transmission is a different story....The Haydens cooler could not keep up even with the primary radiator cooler with my E4OD. I had to purchase the B&M 29,000 BTU Cooler. I drilled the holes bigger and bolted it to the bracket I made... On the hottest days it now runs about 150 degrees or below. The temperature gauge is in the test port.

I also, use a Derale transmission oil pan. Some people say it does not help. Although, In my opinion it does. It takes longer for the fluid to get warm...Downside is it takes longer for the fluid to dissipate heat. That's why they have hollow tubes that run the entire length of the transmission pan that allow air to flow through them. I noticed about a 10-20 degree drop in temperatures with that pan. Depending on outside temperatures...When it's freezing cold I cant get the gauge to buzz past 80 degrees... It also holds about 7qts more of fluid. In my opinion the transmission oil cooler helped out more then the pan and the factory pan works fine.

What I mentioned is over kill. Although, for my application it is a must to maintain a decent temperature. Since I upgraded to dual exhaust it made the transmission run warmer having 2 exhaust pipes. One pipe running down each side of the transmission.

I also wrapped my exhaust pipe going down the side of the transmission with some 2.5 inch exhaust wrap that summit carries that uses metal straps to hold it in place that helped. I also wrapped the Transmission lines going in and out of the transmission with some foil tape were it was about an inch from the exhaust. Going all the way down past the long tube headers...

MY future plans are to add thermostatic oil bypass and remote oil filter to the transmission. This way I can keep the transmission cleaner and also raise the temperatures of the transmission during the winter time here in Missouri.

 

"The ideal operating temperature for automatic transmission fluid is between 175 and 225° F" A reference you can find about anywhere. Transmissions can run too cold which iusn't good for them. Hence many newer vehicles (Tauruses, Crown Victorias, as Ford examples) have thermostats inline that don't allow transmission fluid to circulate through the radiator until it actually needs cooling. In pursuit of better cold weather performance and fuel mileage some very late model transmissions (Dodge trucks in particular) actually have heaters in the transmission to help get the fluid up to temperature quicker. You can score those Taurus thermostats from junkyards for almost nothing and retrofit one if you're good at cutting and flaring steel tubing.
Every factory "towing package" added external cooler is plumped in line with the radiator. The coolant in the radiator also helps warm too cold ATF and regulates the temperature. You can score a factory Ford cooler from about any similar year truck (and Bronco) to yours in the junkyard pretty cheap and they work well for light duty work. They bolt on instead of plastic ties which is nice.
So. The best way to tell if your transmission actually needs extra cooling is to install a transmission fluid temperature gauge. THEN take extra steps to add cooling if needed.
The lines and the cooler in the radiator are too small for motor oil use. You're not the first to think of that. Another low budget way to go is to get the oil filter and and lower radiator hose from like an F350 with a 5.8 engine. This will get you a genuine Ford oi to water cooler. These were used on 1990's-2004 or so Police cars but wouldn't be a direct bolt on to your 5.0 like the 5.8 truck version is.
Most people don't really need an engine oil cooler. A nice big new aluminum radiator will likely cure most of your cooling problems.