With the alt on the driver's side, easiest way is to buy a kit for swp sbc from one of the Chevelle suppliers. You might find a complete intact donor, but it's not likely these days, as the cars are now 44 years old. You will need the pump, brackets to the block, possibly the crank pulley, p/s box, lines and I'd even grab the bolts, as well as the pitman arm.
Devin