Yup!!
I agree,it could be ignition--weak spark will snuff out under a load..
I had borrowed my dads Chevelle wagon years ago--got 15 miles away,and I stopped for a soda..got back in,it started fine,ran perfect!--but the minite I shifted into drive and tried to give it any throttle,it would sputter and stall!--put it back in park,it started and ran perfect again!--but I could not drive it out of the parking lot!...Luckily I was near a parts store about 1/4 mile away--I bought a new distributor rotor,when I looked at the original one it looked like a dime store cheapie,and had carbon tracks all over it(this car had points!)...bought a new rotor,put it in,and it not only fixed it,my dad said the car never had that much power since he bought it!..
It could be a weak coil,module failing if its HEI,or low voltage to the pink wire at the HEI distributor..if you added HEI to an old truck that had points,you need a 12 gauge hot wire to the fuse box with power only when the key is "ON"...if you use the OEM coil wire,its a resistor wire that only has about 9 volts at reduced amperage--and HEI will only start and run at idle and no-load with that kind of voltage..
Might see if your EGR valve is hooked up right too(if you even have one!)--if the wrong vacuum hoses are connected to it,it will open at the wrong time,and could stall the motor out..
Check the fuel pump pressure and volume too--it might be getting only enough fuel to idle at no load due to a bad fuel pump,clogged filter,or a stuck float..good luck--sometimes rebuilding the engine is the EASY part...getting all the bugs out can be more difficult!..
