Maybe I missed something here, but I would like you to clarify a couple of things here. Exactly what kind of car do you have with the weak transmission? Is it a G body, or is it something else? Did this car get an engine swap? Was the engine swapped out of the Brougham? By Brougham, do you mean a Cadillac Brougham? What year was the Brougham? Were you really running 10W30 engine oil in an automatic transmission? If so, that may be at least part of the reason why you're looking for a transmission now. Automatic transmissions are meant to use the correct type of automatic transmission fluid for the particular unit that you have. Automatic transmission fluid is formulated differently than engine oil. Its usually lower viscosity (thinner), and has a higher detergent level. Clearances in the valve body of most automatic transmissions can be in ten thousands of an inch, so any kind of fluid that's thicker than transmission fluid can eventually harm the transmission.
At any rate, if you're looking for a transmission, you should try to find a transmission which matches the one that you're trying to replace. Installing a mismatched transmission will probably lead to the necessity for more modifications, and probably lead to more problems. If all else fails, I would consult someone knowledgeable, maybe talk to a local transmission shop to see if they can point you in the right direction. Good luck.