SB 727 Trans issues

Hi,
Looking for input on flushing trans without removing.
Put this anonymous trans in only to get 340 motor in.
Once the oil got in the converter it started working.
Just drove it about 500 feet to a different building shifted fine.
Went to move it recently to put on trailer and nothing but slip no matter how long warms up.oil level fine.
I did change filter before install. Only thing Im thinking is debri/crap in old oil clogged up
something in valve body??? I did band adjustment, no difference.
Im guessing early 1980's on age.


Thanx in advance.