I started working for the USPS back in 1957. The mail delivery fleet was flat head Dodge, 6 cyl Ford among others. They were being run with low temp thermostats, and who knows what for oil before I started. They went to 180 stats MIl 2104A oil (the first HD oil) and dislodged some quite somesludge in those engines, I'm sure. When you would pull one down for a valve job you still had to spend quite some time to see the springs, however we didn't have any screen plugging.
If that were my Ford truck I would add a quart of ATF to the present oil run it
for an hour and drain it. Refill it with some 10/30 and change the filter if it has one and run it for a few hundred miles then repeat the exercise and run it a thousand. I would also equip it with a bypass filter if you can find one if it isn;t there. A couple of hurried oil changes will clean up i a lot of the mess which I'lltell you I don't think you'll find to bad on a flathead V8. You can see the inside of the engine real easy as the intake comes off in 20 minutes and the major area where sludge will accumulate is in front of you. There isn't much room around the timing gears.