Some thoughts, just to point out things you may not have considered (it's common):
First of all let me say the exact reason why I want to keep the gasoline part is because I won't be able to register a fully converted car.
Can you register (or afford to register) a DIY-converted car? Some localities have pretty extreme requirements for proving it meets various standards, including paying large sums of money to special engineering test firms to verify this.
- I would like to keep the passenger area unused except for minor things, and use the trunk of the car instead in which I can sacrifice the spare wheel.
Can the rear suspension take the weight of the battery, controller, motor, etc that would be placed on it in the trunk, and still perform as you need it to?
I would even drop to 60km as a maximum range to get there
Depending on the speed you go, the number of stops and starts, the amount of acceleration you require (how fast you need to reach your cruising speed), the terrain, road conditions, vehicle + everything else weight, wind conditions, vehicle aerodynamics, etc., it could take 100-300wh/mile (or more) (63-187wh/km) to drive around under electric-only power.
Assuming a 187wh/km situation, that's a minimum of 60km x 187wh/km = 11,220kwh of battery capacity. That's assuming you always use the battery from completely empty to completely full, and it never ages losing capacity, etc. Safer to add at least 20-25% more, minimum, for 14,025kwh, which means you aren't running the battery as hard and it will last longer and give you taht range for a longer time.
I have a 2kwh battery made of EV-grade EIG cells, without any casing around it, that is approaching 40lbs, and the size of a small stack of hardcover books. A 14kwh pack of these, with no protective casing, etc., or method to secure them in place, would then weigh about 14 / 2 = 7 times that, or 280lbs, or 127kg (about two full grown adults), and be 7 times that volume. A casing to secure and protect it would probably add at least 20lbs, probably more, plus some volume. (possibly more if there are significant regulations requiring more protection for it to prevent crash fires).
Either of the rear-end motor drive systems you're considering would probably work; keep in mind they will add all their weight to the rear suspension as well.
Any front-drive system would move the weight up there, but because the engine is there, too, may complicate the design and installation.
Note that in some places, any modification of the body of the car (like cutting holes to pass drivetrain components thru) may either prevent registration or require some form of engineering recertification of the car to do so.
These kinds of regulations kinda suck for DIYers, but they've brought some projects around the world I've seen to a halt, or delayed them for years.