manifold heat control valve

anyone with a slant six 225 answer my question do you need to have a heat riser working .i seem to be losing heat out of the shaft going back against my fire wall .can i weld a steal o-ring in that shaft area to stop the air leaking and is this okay to do.i've been told that you dont need the heat riser working some guys blank it off is this a good thing or will it cause problems.the other trouble i have is that if i drive say for a hour and shoot off the engine say for at least half a hour she is hard to start what is causing this to happen.i;ve just changed my 225 to the( super six slant 6 225).shes a 67 dart.
thanks
chris in canada...