Help! 1969 GMC Pick up needs a new engine...

[MMS1781]

I am having my newly purchased 1969 GMC 3/4 ton pickup worked on by the auto teacher at my school...I am a teacher....when he first looked it over, he said that he would recommend putting an entirely new drivetrain in. I am very new to restoring/working on trucks, I have just always loved them and came across this one for a good price so I bought it. The auto teacher told me to jump on some forums and get your advice for what type of engine I can put into this truck. If I can find one at a junkyard instead of buying a new one, that would be great...but if anyone has any advice on what type of engine woud for sure fit, I would greatly appreciate it! Thanks in advance!