p38 gearbox problems

hi all, i have just rebuilt the engine on my 1999 p38 4L auto and put the engine in but for some reason the gearbox wont engage any gears.

when you put it in drive or reverse it just makes a grinding noise but dont move.

The gearbox is lined up right as all the bolts are in fine.

Any advice or help welcome

cheers, John

It is filled with the correct level of ATF right??

-Wills

Did you make sure that the torque converter was fully home before mounting the engine as they go on with quiet a clunk (mine did any way) as they seem to go on in 3 steps