It will take IDK maybe five times as much power to, by hand, rotated the trans output shaft in the reverse direction, than in the forward direction; this is normal.
I cannot, in my mind's eye, see how the park rod can be installed wrong.
You say that you inspected the clutches and the bands, then reassembled.
I'm assuming this means you took the entire front end out of the trans, cuz IDK how else an inspection can be performed.
if the gear train stayed in the back of the trans, then it's endplay has not changed.
Thus if the too-long fault is inside the trans , Most of the time it's because the rear clutch was not fully engaged on all 4 or 5 clutch plates. So then, when the pump goes in, and you snug the bolts down, you just bend the last clutch which then drives the front end-play to zero, or less. I have lost count of how many times I have seen it.
The endplay spec for the rear is not much, so even if the front is jammed into the rear, normally, this does not or should not affect you driveshaft installation.
But it sureaschit will affect the manual rotation of the output shaft. As will too tight bands rubbing on the drums.
IDK how the backend can be assembled too-long and the snapring still be properly installed. Excessive end-play sure, but not pushing the output shaft back.
As for the convertor affecting the driveshaft, I cannot make that connection.
I could sooner imagine that something got between the bellhouse and the engine, during the mating.
BTW, when you went to bolt the TC to the crank, you had to pull the TC forward out of the trans some quarter inch or more, to where the flex-plate was ............ right?