So I just installed a new engine in my 1992 Subaru Legacy wagon after the old one threw a rod and I've been having transmission issues.

As the title says, I've swapped out my engine for a new-to-me engine from a
junkyard. Anyways, the engine runs, quite well actually and that's not where
the problem I have is. As it turns out, I ended up not hooking up the lower
transmission cooler line to the radiator and I lost a good amount of tranny
fluid on startup. After filling up what I had believed to be the amount lost (I
had the thing up on stands and I'll get back to that in a minute) Anyways I
started it up and pulled it off the stands and although I noticed it took a little
long to get into reverse I wasn't entirely worried as I figured it was just an
anomaly. I checked the level and although it seemed okay, I think I was just
getting the extra from the fluid I had put down the tube. Anyways, I went to
drive it and the first stop sign I hit, it took a full 5 seconds after taking my foot
off of the brake for it to engage first gear and go. After checking fluid levels
again, I discovered that it seemed a bit low so I started to gradually fill it,
slowly pouring in about 2/3 of a quart in pretty small increments and driving it
short distances to let the transmission to actually start using it and it seemed
to be helping. Each time I went further before it would slip. After I noticed a
slip I would bring it back and check the level. Eventually I had it filled to the
third mark and took it out again. I was able to drive around in my
neighborhood for a while, starting and stopping with no issue. probably drove
a total of half a mile before deciding to come back home. I started to pull into
the driveway and it slipped. Anyways, after getting it on I noticed a leak
underneath that seemed to be power steering(the pump was low and it uses
the same ATF as my transmission does) So I went ahead and checked the
level, it seemed like it might have been a tad low so I put a bit more in and
pulled it down off the stands. I went to pull out of the driveway again and it
wouldn't shift at all. Not into reverse, and not into first so that I could pull it
back up on the stands. I think the leak that I was attributing solely to the
power steering might have been actually from the transmission and caused
by overfilling. I've heard that overfilling can cause leaks as well as damaging
the transmission. Being that the transmission should have enough fluid, I feel
like the fluid being low is no longer the issue. Could it be the torque
converter? I didn't pull the torque converter off of this engine considering that
both engines are the same and this one had around 40k miles less than
mine, but perhaps there was enough of a difference in them either from the
wear or maybe just slight machining imperfections. Or is it possible that I
ended up damaging the transmission? Being that the fluid level ended up
being higher than what it was supposed to be and the amount that has
leaked out seems to be quite allot.(however the level on the dipstick doesn't
seem to go down, even after letting it sit for an hour nor after I started it back
up and shifted into all the gears from park all the way down to 1st and all the
way back up again a few times.