engine runs fine for a few minutes then stalls

[lilpup69chevy]

i am curious to anyone who has had a problem like i'm having on my 68 chevy i just bought with a 250 I6 3 spd truck, anywho on the way home it ran fine till i got about halfway home with it then it started cutting out and acting like it was out of gas but it had half a tank, well left it went to the shop to get some tools took off the fuel filter started up ran a little rough at first then it ran fine the rest of the way then died in my yard, well the things i've done its not a carb problem as the choke is not sticking, no vaccum leaks at carb, tried another carb that i know is fine still does it, it really acts like it is starving for fuel, but there is fuel coming out of the line when you crank it, is it possible for it to get weak as it warms up or is it just an ignition problem perhaps? it has me baffled at the moment because i've checked most of the basic stuff any tips or ideas are appreciated thanks