After completing the gearbox assembly and putting back the housing bolts back it won't turn, As soon as i loose up the housing it starts rotating.

 

Did you happen to change any of the gears out or replace the shaft shims? If so, did you measure the original length of the shafts including the stock shims for comparison to what may have been replaced? You may have the cases compressing the shafts on its bearings so tight it won't rotate..

 

Really strange i remove the interlock bolt and torqued out all the bolts of the casing now its working without any issues.

Strange, Isn't it ? now the challange would be to install this interlock bolt back again and see what it does