At high speed 50 mph, shifting into 1st make grinding noise, could this be BAD syncho or clutch not fully released?
No. 50mph is way too fast to be trying for 1st gear... the syncronizer simply can't give you that much leeway.
If the 1st gear syncronizer was bad you would know it by grinding or being touchy when the car is at a standstill. A syncronizer simply assists in the meshing of the gears. The stiffness you refer to may simply be a cold tranny (cold/thick oil). Or (since you said it was a new tranny) it may just be fresh internals and tight clearances.