Yea, that's not right... I have the same set uo, but with some smaller 30's on mine for now. I can pretty much hold 70 on the highway in 5th with the same exact set up... I even hit 85 once (once!) But she's a bit slow, but I am fine with that really... she's not a race car! However, I have no problems keeping up with cars driving around me.
For what it's worth, I am also running a 4.0L TB and a 2.5" PaceSeter cat back exhaust. Those two mods did help quite a bit with the highway end of things.
Both will effect your power. Running too cool is the fault code. The temp sensor and tstat should both be replaced. The fuel pressure at 40 running. Was that with the vacuum line disconnected? Did you check to see if you held pressure above 20 psi for longer then 20 minutes when shutting it off?
I swapped out both the tstat and the temp. sensor last week. I was gonna drive it to get the tag and it ran hot about a half mile from home, let it cool down and I drove it home and swapped out the TStat and the temp sensor because it was showing it was still hot on the gauge. I put the new sensor in and now it doesn't move from cold temp reading. I didn't reset the codes by removing the battery cable, I'm going to do that right now and re read the codes and see what happens
34 PSI with key on and not running
40 PSI running and what looks like 42PSI with the vacuum line off the fuel regulator.
also noticed that @ 3000RPMs the motor sputters and spits, she acts like she doesn't wanna go any higher.
Wrong temp sensor the one you want to replace its the one near the tstat. Your fuel pressure is high too. If the guage is correct. Since you are 10 psi high check to make sure you have good vacuum on the line going to the regulator.
I drive it around the neighborhood and still the same thing, kind of like its hitting a rev limiter at 3000rpms , spits and sputters. I'm gonna go and get a new tstat, engine coolant temp sensor and the other temp sending unit. Anything else?