What would cause a floor shifting Powerglide in my '67 to shift stiffly? Is there some linkage that needs to be adjusted?

Shift swiftly ?

Is it shifting hard into the next gear, like somebody kicked you in the kidneys? If this is the problem, it may not be a problem if the throttle is kept firmly planted or at wide open throttle.

However, if driving normally, what's that?, a smooth shift, barely undetectable, should be felt. You might check to see if the modulator diapharm is broken. Raise the hood and between the carburetor and distributor is an adapter screwed into the intake manifold. It will have, at least, one hose coming from it and connecting to a steel hard line disappearing to the firewall. Disconnect the hose and apply suction onto this hard line, there should be no leaks, as in sucking a flat balloon.

If air is drawn in, then replacement of the modulator valve is warrantied at a repair shop if you decide not to do it.

Remember the hard line disappearing at the firewall? The other end of the hard line is on the passenger side of the transmission attached to the modulator valve. There is also a section hose coupling the hard line to the valve. This hose may have deterioated and in need of replacement. If so, replace with same like and kind of hose.

Sorry. Actually what I meant was that the T handle shifter itself on the console is shifting stiffly at idle. Not the actual transmission when the car is moving.

Okay, since its 67, auto is shifted by a linkage rod from the console shifter. You could probably take some WD-40 and lubed all the joints.

After freeing everything up, you might liberally spray the joints with aerosol white grease.

Your sig reads like a well-optioned car