Reverse issues on 904 transmission

My 67 with 273 904 and 7 1/4 rear does the exact same thing. Sometimes on first start of the day reverse feels like it is slipping. drive is 100%, subsequent reverses are fine. Trans was rebuilt ~20 -30 years ago and sat for 5 of those years, fluid has not been changed since I got it. (I know I know)

I also have the clunk. Not sure, but I attributed it to the diff being OLD (350,000 miles) and loose. grab your drive shaft with the car in neutral. (Wheels chocked and E brake on) and see ho much rotation you have. My guess is that when you park the pinion is still touching the face of the ring gear and when you put it into reverse it freewheels till the teeth make contact with the back side of the ring gear and clunk!

Admittedly I don't know all that much about the nuances of the 904 so maybe it is a trans issue causing the clunk.

I'm sure we both could use a rebuild!

But at least your not alone in clunking and slipping in reverse!