The 390 is not a torque monster. It's a slug. If you want a heavy motor, get a 460. The 390 is not worth thinking about. If you got it in there you'll be doing a lot of bragging and a lot of excuse making. You get a 460 in there and you'll have no need for either.
The things that made a 391 different were things that made it able to survive in a hard environment. It was less powerful than a 390, but more durable. Full skirt pistons (more friction) tiny valves and ports (more airspeed at truck rpms but less power--these were governed to 3600rpm) forged cranked (you need the flywheel to balance it, and that is probablt 40# or more so it doesn't stall when you let it up in a loaded dumptruck, meaning less acceleration if it;s installed in a light vehicle). There are other differences and I can't remember them because I am confusing them with the Dodge truck engine I recently go rid of. But I think there are some collant passages added so it's possible the intake manifolds are different. And the exhaust manifolds are certainly different.
They are based on the same engine, but there are not a lot of similarities when it comes to exchanging parts. And for what it's worth, there was an FT called the 389 as well, and it was also a 390 based motor.
I used to have an LN700 with a 361FT so I've been down this road and probably still have the serivce manuals and that Steve Christ book if my wife hasn't donated them to the public library.
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