What's the cheapest way to install and exhaust system on a big block a body

If you use headers you could install an X-pipe mufflers and order some tailpipes.
Summit Racing SUM-642125 Summit Racing™ Universal X-Pipe Kits | Summit Racing
Flowmaster 15826 1963-74 MOPAR A-Body 2.5 Inch Pre-Bent Tailpipes and your choice of mufflers.
Or skip tailpipes and exit the side.