When I am going down the road at about 3000 RPM, I here a rumbling noise the is cyclic.   I seems to get worse when I decelorate.   What should I be looking for?   I have a 273/904, TCI converter.  

Retaerd your ignition timing 2 degrees and readjust your carb.  

Have you checked for loose converter bolts?
Or possibly loose exhaust?  

Alright all of us will give you a different answer and you decide which one makes the most sense.
New u joints & balance your drive line.
A lot of it has to do with the pinion angle, see when the springs sag or new springs rake the car too much the angle changes and can cause vibration/noise, excessive u joint wear.
Your welcome.