What transmission and rear end in your truck?
What is your average speed on the freeway?
What year 261?
All of them would have had a single barrel Rochester.
It has a mixture adjustment screw (here is the simplest technique, without any other gauges/testers/equipment):
- run it until warmed up at idle
- adjust the idle speed to a slow but steady idle (500 rpm)
- adjust the mixture screw inwards until the engine gets rough
- adjust the mixture screw outwards until there is no change in engine smoothness
Step 1 and onwards at the lower right of this page. That gets you a rough carburetor adjustment.
If you have a vacuum gauge, you can attach it on the port on the side of the intake manifold under the carburetor. Loosen the collar on the carburetor and rotate it until you obtain maximum vacuum. That gets you a rough timing adjustment.
