According to my research, every EV project has to do a cardboard mockup of the engine and battery placement. The point of the mockup is to get a good idea of what is going to fit, and what is just not going to work out.
Even though I'm planning on replicating the frame in a solid modeling program, I thought it was a worthwhile exercise, revealing a couple of potential problems. Note the can of Sherwin-Williams finest representing the motor (same diameter as a D&D or Advanced DC).
My first cardboard revelation was that it's a lot tighter fit than I thought it would be. I modeled the 60AH ThunderSky batteries, but they are a smidge too big to fit in some orientations. The second eye opener was that a few of the original mounting lugs will need to be ground off, to let the batteries sit higher. I hate to grind anything off, but there is just no way to utilize the space between the frame rails without doing it. I think the remaining lugs will be sufficient to hold the motor and battery frame.
The motor will fit decently, assuming I use an ADC or D&D type. The paint can is 6.75" in diameter, compared to 6.7" for the D&D. An Etek-RT will just make it, but it will be a tight squeeze at 8.07". There is no way a Perm132 will fit inside the rails (8.75"), and I'm not wild about mounting the motor outside the cage.
I'm planning on mounting the controller and other electrical connections between the upper frame rails. The hollowed out gas tank will cover everything nicely, but may need some venting, or a fan to keep everything cool.
The prolific use of duct tape, although not permanent, would make Red Green proud.