My car doesnt start need some help?

i have a 1948 plymouth 4 door with a 318 v8 its a 12 volt system... i just changed the hoses put a new battery, alternator, change the spark plugs to some platinum, new cap and rotor and wires. new msd blaster coil fuel pump fuel filter, re grounded the battery and grounded it to the body... the car takes a while to start but doesnt want to stay on unless i have my foot on the gas pedal the carb was tweaked with.. now what do i do?