When a transmission is removed or engine is pulled out, yes the rear crank shaft main seal is an inexpensive part to replace and a HARD to get to part if its starts to leak
New torque converter is recommended
You can only use a 2001 to 2011 5R55E from another 4x4 for this swap
If you haven't swapped in or installed an automatic trans before there is one BIG THING you must remember to do
SEAT THE TORQUE CONVERTER INSIDE THE BELLHOUSING
FIRST!
With transmission ready to go in, put torque converter in to bellhousing on to input shaft
Rotate torque converter while pushing in
It will slide in when splines on input shaft align and THEN keep turning and it will side in more when pump slots/tabs align
Then turn it a few more times to make sure its all the way in
Should be touching back of bellhousing or very close to
Then you can mate transmission to engine, it should slide on flush, you should NEVER EVER have to force trans and engine together, something is wrong, STOP and find out what is wrong
Once engine and trans are together you slide torque converter forward to bolt it to Flexplate
Short video of seating torque converter
If you don't do this you WILL ruin the transmission, ask me how I know, lol, I ruined my Fresh Rebuilt trans on my first automatic install