GM-GMC: Stalling, side valve, leak check

Question
I have a 1979 gmc k15 stepside, I just put a rebuilt engine in it from S&S engines, its a stock 350.  I was driving home and stop at a light the truck idled for a while and then it sputtered and stalled. I poped the hood and a hose had come off the driver side valve cover I put it back on the truck still did not start, I then pulled the air filter off and with some time and allot of pumping of the gas pedal it started I stopped at the next light and same thing, this time I just pumped the gas and the truck started and got me home it had no problem idling once home and parked.? If you could help thankyou much.
Andrew

Answer
this is my take on this... you have no back pressure which is usually caused by an exhaust leak... check the exhaust manifold and the catalytic converter the cat maybe clogged... the manifold maybe be cracked or bolts stripped out .... check for other major leaks in the system... also your fuel pressure may not be good while the vehicle is in gear sounds wierd but honestly everything in the engine drops pressure when it is put in gear and if it is so low it will cause it to stall or sputter upon take off or when you apply the brake...