What year is your 391, please? Of course, that does make somewhat of a difference in its power ratings.
If I remember correctly, 1974 was the last year for the 391. It produced 180 HP @ 3600 RPM. It produced 320 foot-pound of torque @ 2400 RPM.
A 1974 534 Super Duty produced 213 HP @ 3000 RPM--the speed to which it was governed. It had 393 foot-pounds of torque @ 1800 RPM.
rusty70f100 is correct in his post about the 391 being so closely related to the FE motors. The 361, 389, and 391 were known as FT engines. Basically, they were lower-compression, slower RPM, high nodularity iron crankshafted torque motors.
The FE's began as torque engines, too, for light duty pickup and station wagons and land yachts. Ford really made them scream, though, especially with the 427 series and the 428's.
Many of the FE parts will work on the FT's.
The Super Duty engines were mainly slow-RPM torque monsters. They were built to pull hills with heavy weights and work as huge stationary engines that could run saws, pump water, and move conveyor builts forever and a day.
Even if you disable the governor on an SD, they will not wind as tightly as an FE or FT. And, to be honest, you wouldn't want them to do so.
Very few Ford engine parts are cheap; still, you can still get parts to make an FE motor run like a striped ape! I personally know of no source for SD parts except for some marine and industrial applications anymore.
One Ford engine that almost everybody, by the way, has forgotten about was the old 572. 572?? Yes, Ford built them back in the Second World War and Korea for Uncle Sam to use as tank motors. Tractor pullers grabbed gobs of what was left of them up back in the 70's and very early 80's for pulling engines. Several companies made hop-up parts for the 572's for a few years. However, today, I don't know of anyone who still does.