The only problem I see with using an AC condenser is that the coils may be too small and restrict the oil flow from the transmission, causing too much pressure drop. Check the oil pressure at the transmission to make sure the cooler isn't restricting it. Too little pressure might make it slip or cause funky shifting problems.
A good cheap cooler would be to find an oil cooler off a motorcycle, junkyard Porsche, SAAB, or BMW. All '88-92 BMW 325i's have a nice rectangular oil cooler mounted right below the radiator that hooks up to the engine oil and flows pretty well. They are easy to get out if you can find one at a U-pull-it, and they rarely go bad.
