727 with 3400 stall will not idle in 1st?

hello,

I have a 71 dart 340 that will idle and run perfect in neutral but when I put the car in first it drops the idle and stalls out. if I two foot the car it will continue to run fine. Should the car be able to idle in 1st gear or should I be shifting it into neutral at all stops? Thanks in advance for any help you can give me.

Bryce