hard clunk in reverse??

good morning.

ran across this site recently and signed up. what a nice site. sorry i missed it all this time. with all the knowlege on this site i have a question regarding my 727/8/34 driveline in a 1972 duster 340. the tranny has 2 quirks. the first is when cold (bellow 40 degrees ) it will not go into reverse until warmed up. the second is a very solid clunk when i put the car into reverse. this is only reverse. now im not sure if that is a rearend clunk or tranny. hard to tell. the u-joints are brand new and the car is rock solid with a ground up resto. what do you guys think on both?? thanks for the input and glad to be here.