Terrible clunking noise when shifting

[Jerebus67]

I just bought a 69 RS SS 350 camaro with a M20. It is the original tranny and rearend but within the last couple of years has been totally gone through and rebuilt. There is a terrible clunking noise when shifting in 1st, 2nd and sometimes 3rd gear. (ok in 4th)
One garage said they just thought it was body flex but there is no way this is body flex. Any ideas on what it could be? Everything is tight underneath. Everything is brand new underneath.

What other info can i supply to help?