[Ryland]

The interface between the engine and the electric motor is what you are going to have trouble with getting to be smooth and is part of why it took so long for comercal hybrids to come to market, we had hybrids for 100 years or more but they were fussy and jumpy.
But for you to build one you are going to have to have the mind set that you can do it along with either the skills or the patients to do it as you have to chose if it's going to be easy, quick or cheap, you can have one but not all.
It does depend alot on the type of vehicle of course, a truck or rear wheel drive car is going to be easier as you have a drive shaft that you can pull out and replace with a motor, a front wheel drive car is going to be the hardest, but as I said, non of this is going to be easy or cheap or quick.