Since the engine is in the truck, why not just put it in high (3rd or 4th) gear and rock the truck? You will surely get a lot more torque rocking in gear than you can with a wrench on a balancer or flywheel.
Patience is a virtue in freeing up old engines. They took years to seize and may take weeks or even months to free up, but I am continually amazed at what I have seen roar to life after sitting for many years.
A neighbor had an old Case tractor loader backhoe sitting in the "back 40" for years with no can on the straight up exhaust pipe. That pipe caught rain for years and funneled it into the manifold and ultimately into the engine. It was rusted beyond believe when we peeked in the old injector holes (diesel).
After six weeks of dumping penetrating and marvel into the cylinders and rocking it once a week it freed up and do you know a week later it roared to life and produced a spectacular smoke screen at the same time. Five minutes of checking for leaks and warming it up and we drove it out of the brush. Stopped along the way to pull a stump!
It took a little over 6 weeks of weekly oil squirting and rocking but it freed up.
Have patience.
