It really is NOT running anywhere close to great. The compression is way to high for the fuel available today. With the high compression it has to be de-tuned with retarded timing to even run which makes it far less efficient than an engine with the proper compression ratio. The cam is also too large for efficient operation. The manifold is wrong for street operation. The carb is too large for street operation in a heavy vehicle. This combination would be OK for high RPM operation doing weekend racing in a mud pit somewhere or in a Mustang built for the dragstrip but it is all wrong for efficient operation in a heavy truck on the street. As I said b4 it is a classic example of "Bigger is Better" syndrome or something like Desktop Dyno misuse/misapplication.

I had an old FE 390 back in the late 60's with 12.5:1 and some similar specs pushing 500HP and it took 115-130 octane fuel, either Phillips Flight Fuel which was a 115 octane super premium or 130 octane Aviation fuel. Back then it did run great and was a screamer that revved like an Indy car but it only got 4-8 MPG. I would not even think of operating it today.