Bad bad news

[white71chevy]

I need to no what's wrong with my truck. its a 71 chevy with a 305 in it and I'm pretty soon it is the original tranny. So here's what happened I was driving my truck and at 60 it started making this really loud squealy nosie. So I pulled over as soon as I could I got out looked at everything I thought it was the pully. The pully looked fine as did everything else. So I got in it and put it in gear and it was like the tranny didn't want to pull the truck into a roll so I gave it a bit more gas and it moved. After it started moving the tranny did fine no nosies or drag I only went like 20 tho. Idk what's wrong with it is the tranny going out? Can it be saved? Or do I have to buy a new one? Also the truck was sitting for 5 years I got it running a few months ago and I drive it as much as possable it has made this nosie a few times but I thought it was just the pully. Its running great has no problems with all trips or anything its just that nosie and now the tranny. I need to know what's wrong I have been working all summer to drive this truck my senior year it starts in 2 weeks. Sorry for long post I'm just really weird my truck is messed up I love this truck its my baby lol plz help.