Just make sure you have good fresh fluid at the right fill quantity and beat the crap out of it. I'm guessing from your username you have a 2004. This means you don't have a 4R70w, you have a 4R75w which is much much stronger. Even a regular 70w would be totally fine but a 75w doesn't even need to be upgraded if you do a built motor with a power adder. People routinely run them up into the 600rwhp+ range with no issues.
So yeah you are good to go for a long time. The 2004 Mustang
GT/Mach1 automatics had the strongest automatic transmissions out of any of them until the 6R released in 2011.
2000 Mustang GT Steeda #0048 34k miles. 99 Cobra 4V swapped, FTBR IRS swapped, exhaust, tuned by James Gordon.
Just because I give you advice, doesn't mean I know more than you. Its just means I've done more stupid ****.