If you already have the crossmember off, just go ahead and lower the rear of the transmission down some (assuming you're supporting the engine with a jack), and you should be able to get to the top bolts with some extensions. I've never torqued them to spec because they're so difficult to reach, and in the last 9 years that I've had Mustangs, I've never had an issue on my own, or anyone else's resulting from improper torque specs.