Yes,I think the linkage is the problem and yes the '70 linkage should fit.
tactransman: You were 100% correct. I got a '70 340 Six Pack kickdown linkage from A&A Transmission and installed it today. Like all my installations, it got exciting real quick. When I got it out on the road, I floored it and the console shifter moved out of drive to neutral. %$^%!! When I got home, I saw that the kickdown linkage hits the drive selector lever when the car is floored. It pushes the selector lever from drive to neutral.
I looked at the service manual and I soon discovered that the rod from the floor shofter to the shift lever on the transmission was backwards. I guess the rebuilder didn't know how it went and it shifted just fine (until these other problems...). I turned the rod around and it shifts perfectly now--downshifts, upshifts, perfectly.
Because everyone seems to like pics so much on this site, I have enclosed two pics of the new linkage and a no-so-good shot of the throttle rod at the transmisison lever. Maybe this might help others with similar problems, but you are THE MAN! Thanks again to you and everyone who weighed in with great recommendations!
Of course, one more bit of "excitement." Once everything was working, I decided to get gas. It overheated. I think I diagnosed it right as a stuck (closed) thermostat because once I changed it, it was fine. So much for those balance sleeve "high-performance" thermostats, eh?