E-Muscle Cars has developed a heavy duty rear-motor EV conversion package for most 1971-1996 GM B-Body cars that use the standard triangulated 4-link rear suspension.  This allows you to install an OEM Tesla model S large drive unit where your old rear axle and differential currently reside.  This is by far the most space-saving way to convert a solid-axle car to a high-powered EV, leaving the entire engine bay, transmission tunnel, underbody, and trunk available to put batteries.
 
The package includes a heavy-duty steel "De Dion" rear axle, adjustable 4-bar links (featuring fully adjustable johnny-joints), hubs, wheel bearings, Wilwood 4-piston brakes, base used Tesla LDU motor,  rear motor mount, front motor mount, Tesla LDU wiring harness, EV Controls motor control unit, throttle pedal, brake pedal switch, and other critical EV conversion components.
 
Custom motor mounts, rear shocks, rear springs, rear CV axles, electric parking brake and upgraded Tesla motors are available at an additional cost.  Installing motor mounts will require some drilling into the frame rails, and may require welding. 
 
Batteries, charger, contactor box, fuse & relay box, and HV wiring can be purchased separately.