Is it better to to put a rebuilt 350 in my pickup or buy a used 350?

I don't know what is a better route to go, I know the this truck has new tires, tranny, windshield alot of other new parts. I found a good running 350out of a 1991 k5 blazer for about $300 the other day and know someone who can put it in for me. But on the other hand a rebuilt 350 costs about $1200 which is kinda steep for me and don't have a lot of extra time to be messing around on projects like this. I really like the truck and want to keep it in the family. What should I do?