My truck is stalling (carburetor)?

Ok so this is the truck that I am working on (i'm 14), I have a 1966 mercury half ton, and I got it running but after about 2 minutes of driving it (around the yard of course) it just randomly stalls, and it is in gear and i'm not shifting or anything (4 speed) just driving strait. any ideas why its doing that. oh yeah it doesn't have the original 352 my grandpa switched it to a 390 big block before I was born, thanks.