So what do you have to do to convert a gas powered car to electric? Here's a moderate summary. I plan to have a detailed post on each of these sections as I get into them.
First, get rid of the engine and anything related. On my car it was fairly simple, mostly because of it's age (no computer = less wires) and the fact that it's rear wheel drive (more space under the hood). In addition to the engine you will remove the exhaust system, fuel tank, fuel lines, radiator, and anything else that's not serving a purpose. On my car there were lots of heat shields for the exhaust since it runs very close to other parts of the vehicle that don't like to get hot.
Ok, now your car is stripped down to just the essentials. Since the motor is the only part that has to go in a specific place, it's the next logical step to mount your new motor. You'll have to make, or pay someone to make a shaft or flywheel coupler and an adapter plate. These are precision parts, so if you don't have the right tools or skills you will end up with lots of vibration (at best) or a broken motor and transmission (at worst). You will have to fabricate some motor mounts as well.
Next step for me will be to make a bracket for the air conditioning compressor. The compressor on this car is equipped with a V belt pulley, so it's a lot easier to find the right pulley for the motor and the right size belt. The motor has a shaft on both ends of the motor for just such a purpose. More on this later.
Now you'll reach the point when you feel like 90% of your time is spent gazing into the open space under the hood. You need to plan out where every last part is going to go. With careful planning you'll minimize any rework, though no matter how careful you are you'll reach the finish and think of quite a few ways it could have gone together more efficiently. By now you should know exactly how many batteries you plan to use, what size and weight everything is, and how you need to distribute the weight. My plan is to build up weight under the hood equal to what everything was before I took it out. This will minimize issues with alignment, steering, and braking. Make some cardboard mockups of everything and see how it all fits. Once you have a firm plan in place it's time to start the major custom fabrication. Lots of brackets, mounting plates, enclosures, etc. Your welder and abrasive blade chop saw are your new friends.
With all the brackets made, it is time to mount all the hardware. Controller, DC-DC converter, relays, contactors, pot box, fans, fuses, vacuum pump and reservoir, batteries, and of course, lots of wiring.
These are the basics, but there will probably be many other projects to do that will be vehicle specific. My vehicle needs quite a few areas of non-conversion related attention like new seats, bleeding the brakes and replacing a broken bleeder screw, loose wheel bearings, broken sunroof hinges and latches, and a few cosmetic things. I'll also have to redo wiring for the air conditioning and heating system that will no longer function the way they used to. I also plan to add some custom instrumentation. If the car had a computer, I'd have to trick it into thinking everything's running the way it used to, like compensating for missing sensors.