I forgot to ask if the car was running when you did the hole overflow test?
Are you sure that the lever mechanism between the shifter and the transmission is working?
I mean page 7-168 of the 1999 Factory Service Manual lists 25 things to check for
no reverse or slips in reverse.
So, of the things listed as one of the causes, low fluid and low line pressure are the most likely given what you did. It's always possible that the new filter is somehow clogged.