If it only slips in first gear it is probably related to B-2 or the valve body.
Remove the rear piston cover on the right side back of the trans.(to do this push the cover in and pry out the lock ring) pull out the piston and see if it has cracked or broken.
If it is broken just replace it with a new one,if not reinstall it making sure the thrust pin is installed correctly,then consider a used or recond. trans.