I know its late in the game, but I have to disagree with several of the above posts.
First putting in an original C4 requires NO FLOOR MODIFICATION AT ALL!! The stock automatic shift bucket assembly is available and goes right were the current stick shift comes out of the floor. Most of these kits will have the rod included. The only issue is hooking up the kick down to the throttle linkage, again there are several kits available to do this easily. <acronym title="transmission">Trans
</acronym> Yoke is different but again the aftermarket comes to the rescue ,you can find adapter yoke that will allow the same drive shaft to be used in most cases. The <acronym title="transmission">trans
</acronym> mount and cross member are identical on the 6 cyl and small block cars regardless of <acronym title="transmission">trans
</acronym> so no issue there either. The automatic brake pedal is available again new in the after market and is simple to un-pin slide it apart take out the old pedals slide in the new one and re-pin it. Radiator can stay the same just add one of the aftermarket <acronym title="transmission">trans
</acronym> coolers and run your lines to it, again the aftermarket has all this available.
The entire swap is a nut and bolt operation, and a fairly competent back yard mechanic can do the conversion in a couple of days with out any welding or fancy fabrication.
Secondly if you go with an original C4 tranny, adding a good after market shift kit, a decent 2000 - 2800 rpm stall convertor and a Hipo servo will give you all the <acronym title="transmission">trans
</acronym> you'll need for the car.
One thing not mentioned above is the rear gear ratios, most if not all automatics in that vintage used either a 2:80 or 3:00 rear gear. Pretty tall i know but it keeps the revs down at highway speed. Most of your stick or manual cars started at 3:35 to 3:50 with 3:79 and higher being fairly common also.
My 65 was a C4 with a built 5.0 in front of it running a equalock 2:80 rear (the 5.0 was stealthed painted to look like a 65 with cobra valve covers.) Any way the car would run at just over 2800 rpm at 60 mph ( a little whinny i know at hi way speed but still livable)
either way have fun if you find another car with the auto you want or a nice car with a manual <acronym title="transmission">trans