what should the fuel pressure be

"Known" for fuel pump problems.

Call your Local Ford Truck dealership they will tell you what it should be at. BUT there is also the problem of it heating up etc. The fuel pump fluctuates right before it pukes. Providing symptoms for all different types of fuel problems.

Been there done that.
Stopped in 5 repair shops while traveling down the road and none of them could figure the problem out. Pulled into Ford and they went right to testing the fuel pump.....and I had to replace it.

I thought you would of had more responses by now. Tons of info at other places that are better to deal with your problem. Not trying to be smart guy or anything, just saying. Do be careful though. On my 2001 (5.4) has no return line and what I found online did. So be careful of what year the poster is talking about compared to what you have. (Especially with spark plugs around your year) With mine the problem was a wire not making good connection at fuel regulator. Loosing pressure overnight might be simple as a cracked gasket or loose fitting. I would start with clean filter before I did trouble shooting.