Need Help: High RPMs on Warm Engine and Trans in Gear?

I have a '68 RS/SS with a 350 engine and TH300 trans. that exhibits the following problem:
On a cold engine start I get an idle around 800 rpm (this seems low as
I think the warm idle spec is 600). The car drives and shifts through its gears fine and I get a steady idle through P,R,N,D,2,1. However once the engine & trans reach operating temp the car drives/shifts fine with a 700 rpm idle in P,R,N but in the drive gears I get a 1200 rpm idle which is putting enough torque on the wheels that you really have to stand on the brakes to keep the car from moving.
The previous owner put a new torque coverter on the car about 9 months ago...
I am struggling with where to start looking between (1) the "low" cold engine idle, (2) the increase in rpms when putting the car in gear, and (3) 1200 rpm translating into enough torque to really move the car.
If anyone has any suggestions on where to start, I would greatly appreciate them.
Thanks.
-John
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could the shifter linkage be pulling on the throttle linkage ? or maybe the left motor mount is broken and the engine is torquing up on its side and causing the throttle to open a little , with out looking at it its hard to tell , what type of carb are you running ? and Intake manifold ...are they stock or aftermarket ? you could try removing the throttle cable / linkage at the carb and see if it still happens ... hope this helps soon
I have a '68 RS/SS with a 350 engine and TH300 trans. that exhibits the following problem:
On a cold engine start I get an idle around 800 rpm (this seems low as I think the warm idle spec is 600). The car drives and shifts through its gears fine and I get a steady idle through P,R,N,D,2,1. However once the engine & trans reach operating temp the car drives/shifts fine with a 700 rpm idle in P,R,N but in the drive gears I get a 1200 rpm idle which is putting enough torque on the wheels that you really have to stand on the brakes to keep the car from moving.
The previous owner put a new torque coverter on the car about 9 months ago...
I am struggling with where to start looking between (1) the "low" cold engine idle, (2) the increase in rpms when putting the car in gear, and (3) 1200 rpm translating into enough torque to really move the car.
If anyone has any suggestions on where to start, I would greatly appreciate them.
Thanks.
-John
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

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