Help asap 350 won't start

[Squareforceone]

I don't know whats wrong with this bastard i just put a new motor (350) in my 84 i went to take it on its first drive since the motor install and it won't start. I got in it today started it easy enough let it idle for a few minutes the thing ran fine then i got it put it in gear and pulled it forward a bit it was still running fine. I pulled it forward a bit stopped to make sure breaks were working then pulled forward a bit more and it started running a bit rough and then died. Now i can't get to run again. I tried a bunch of times i did finally get it to run for a bit but the only way it stayed running was by me pumping the hell out of the gas as soon as i let for a second it died… I don't get it was running fine and idleing earlier today now it won't run. It tries to fire while cranking but won't. Could it be because it's cold outside it is like 17 degrees right now..