1981 F100 4.9 C-6 automatic.

5A6744CC-6E45-4FC1-80E8-0ED74B37C298.jpegJust put a brand new motor on my truck timing is set and and it purrs. Nevertheless I cannot get her to go over 40 MPH. I have disconnected the kickdown linkage and this thing pictured that has a cable running almost parallel to the throttle cable. I don’t understand how the kickdown linkage work or this thing pictured here. Any help would be appreciated

You should look at the page at Bullnose Forum/Forum FAQ's on how to post a pic.  But here's your pic.

And the thing the arrow is pointing to is the speed control, aka cruise control.  It's cable to the throttle can be disconnected until you get the problem sorted.

certainly, we need a bit more info. the diaphragm for the cruise control can be left disconnected for a bit. the kick down lever is a different issue. if you can get a few pictures of the back and sides of the carburetor and linkage that may help.
 aside from that. with the linkages and cables disconnected., verify that the throttle can move a full 90 degrees to wide open throttle. then reconnect the kick down and verify it again. next reconnect the cruise cable and verify.
if nothing is binding or interfering with the range of operation then it's a different issue altogether.
my next thing to look at would be fuel

You said timing is set to what?
If not at 10* to 12* BTDC, vacuum disconnected and plugged, bump it up.

With no vacuum bring the RPM up from idle and see what the timing moves to.
Now hook the vacuum up and bring the RPM up from idle and see what the timing moves to.
Let us know what happens.

You said new motor?
Did you drive this truck with the old motor and if so how long was the truck out of service / not driven?
Was it sitting inside or outside?
Why was the motor replaced?

I had a customers car the motor blew a head gasket or cracked the head, too long to remember now.  
Car was pushed in and out till a new motor was installed.
Pushed it inside to do last little bit and fire it up and could not get it to start, wanted to but would not.

We gave up for the night came in next day hit the key and it started right up.
We also got flooded with water shooting out the tail pipe!
The exh / muffler filled with water and being winter it froze and would not let the exh gases out so would not start. Sitting over night in the warm shop was the fix.

If the exh was left open something could have made a home inside and it is plugging the exh system and not letting all the gases out and why only 40 MPH.
A bad cat thats plugged and or coming apart and plugged the muffler will do the same.
Dave ----

Is the exhaust system free of obstructions? FuzzFace2 gets credit for making me think of this due to his memory of an ice filled muffler.

Removed the cat. Problem
Solved. Thanks

Glad you figured it out. Good to know.

Will someone tell my wife I was right for a change

When I got my truck it only had a cat used as a muffler and when I got it off it was not plugged but was starting to break up and I am sure would of plugged the muffler if the truck had one.
Dave ----

good call.