Your Trek 7.3 looks very nice.
Firstly,do you mind telling me the size of your 8 speed freewheel here?
If you don't know the exact size of your freewheel.
May you can offer me the brand and model of your freewheel?
So that I can search the size of your freewheel online directly.
Besides,here I'd like to give you two suggestions as below:
1).If you use PAPAMOTOR Kit fixed 6 speed freewheel, and wanna use their E-brake handles(E-brake handles can cut off the motor power directly) to replace of your original brake handles.You have to purchase a new separate derailleur for the 6 speed freewheel use.As I know, your 8 speed Deore derailleur is attached with your original brake handles together.
2).If you insist on keeping your original brake handles,you can also use your original derailleur Provided the 6 speed freewheel size is suitable.However,honestly, considering for the safety,usually PAPAMOTOR will suggest every customer to install their E-brake handles.Otherwise,you can use the thumb/twist throttle to cut off the motor power only.It's very dangerous.
Look forward to geting your reply.