May well be a faulty switch , but more likely
the linkage and switch often have to be re-adjusted when a tranny is changed.
If you can get it started by moving the shift lever while attempting to start, it is a simple adustment on the switch. There
is a slot on the fastening bolts that allows the switch to be rotated to the correct posistion[ adjustment] .
You may just want to bring it back to the installers and asked that this be checked.
If you are a DIYer, you will see the switch on the left side of trans, just above the pan at mid-point at the end of the shifter
link. [ which is also adjustable]