If it doesnt slip in Drive then the clutch or band that applies reverse is kaput. (if it was a C4 C6 , torqueflight etc for band. I dont remember if the 350 uses a clutch or a band there but can find out quickly although someone knows here for sure. ) Auto trans are really fairly simple. If it slips in Drive high (3rd) and reverse the direct clutch is beat. If it slips in all forward gears but is fine in reverse the forward clutch is beat. if it slips in reverse only the low/reverse clutch (or band depending on the trans ) is beat. In Low the sprag gives 1rst gear and the band is just an assit for hard work so is often only on in low shifter position and is not used in drive position low. If it slips only in second gear then the kickdown band or clutch (depending in what make of trans) is beat or if it is a band it may just be out of adjustment. Especially if it has a high ratio kickdown band apply lever. Trans are fun to do. In many cases there isnt a single part of the trans proper that will go in the wrong place . I am most familiar with mopar but they all work similar for 3 speed auto trans. The first one is the biggest obstacle. If it works you're away for the rest of your life. If not try again. Clean is big and a manual is required but they are not hard. Be particular though because you dont want to have to pull it again. Don
I would look at the filter & fluid to verify that they are clean & at the correct level...you might get lucky...and if not, see Don's post. A neat summation.