You're keeping all the petrol stuff up front? Is your car front wheel drive or rear wheel drive? being rear wheel drive, and keeping all the original engine and drivetrain, once you put a tesla motor in the back (that'll essentially replace the diff completely and maybe a good chunk of the rear suspension) how to you plan to drive power from the stock engine to the rear wheels?
Second general question, which applies to it being a hybrid with the engine in front and tesla motor in the back, Or if its a full electric conversion, What batteries are you planning on using? What capacity will the pack have and where abouts in the chassis do you think it'll fit?
Answering those 2 questions will quickly show that your idea won't work and instead you need to approach this very differently.
If it were me, I would put the tesla motor in the rear, ditch all the petrol stuff and go full electric, At one time I read an interesting article on this topic, if you are interested visit the website here