Tranny Making Thumping Sound

I Own A 1991 Tristar. I Just Installed A Rebuilt 454. When I Put The Engine Into Gear I Hear A Thumping In The Lower Gear At The Back Of The tranny Where The Shaft Inserts. i dont know much about velvet drive transmissions, but is it possible a barring could have siezed up in there if the boat hadnt been used in a year?