727 transmission dilemma.

I hear you.
As others have said, rotating the output in opposite direction is always more difficult, We all understand that; but that's not your problem.
Here are some possibilities;

1) If the trans is standing on it's face for this test;
gravity will be enough to cause all the clutches to fall together, and when they do, there is enough resistance to turning, to cause concern. This is especially true if the trans has been sitting this way for an extended period of time, which seems to cause the oil to be squeezed out.
But as soon as the trans is horizontal again, the clutches separate, and most of the resistance usually goes away.
2) bands too tight.
IDK how or why this is, but it has always seemed to me, that the L/R band in reverse direction is sortof self energizing. and I think the flex-band is worse for this. The KD band seems less sensitive. Plus when the trans is on it's face, the bands fall down a bit and rub on their respective drums, adding their bit of resistance to the clutches.
3) If when installing the front clutch onto the rear clutch, or
when installing the front end into the case,
one of the clutches fails to engage the splines of it's partner, then friction is greatly increased. and if you assemble it this way, it will bind up, but usually in both directions. IMO, this is not your issue.
4) But, if during assembly somehow a thrust washer to a planetary comes out of place, or is installed in the wrong anchor holes, then the geartrain will seem normal, until you load the front end, and snug it down; that action will crush the tangs on the washer, jamming it up. Depending on your endplays, this could produce a varying amount of preload, in addition to the normal one-way resistance.
The washer out-of-place thing should be discovered while checking the geartrain endplay, prior to loading the front end.
5) And finally, IIRC the sprague race has a lead-in chamfer on one side, that needs to go against the support. It's been well over 20 years since I last assembled one of those. So my question is this, what if it's in backwards? I mean, this will not affect the operation of the sprague, but if it was possible, and I don't know that it is, that the race does not seat, then the entire geartrain would be moved forward. If this was to occur, it might make installing the snapring difficult, not to forget the governor cross-pin. Now; I'm not saying the is your issue nor if it's even possible; I just remember the chamfer. So, if you cannot find an issue elsewhere, I guess a quick peek would be prudent.

IDK which....... if any ....... of these possibilities is your issue; but I've been tripped up a time or two by these kinds of things.

But as mentioned, once the trans is horizontal, a modest amount of resistance to rotation in the reverse direction is normal.

Good luck